I haven't read any of the other comments here so please forgive me if I add anything terribly redundant. Likewise, these are, as always my own personal opinions...please use them as such.

To answer the OP's question, a great man once said, "If it sounds good, it IS good" - Duke Ellinton. To me it sounds as though the OP is trying to base how "good" an individual guitar player is on concepts such as "technical prowess" and that's simply NOT the case. The thing to remember here is that music, as with all art forms, is highly subjective and as such, technical virtuosity isn't really a measure, or not the sole measure, of how good (or how bad) someone plays. Case in point: Yngwie Malmsteen. Yea, the guy can play a thousand notes a minute, but his music ALWAYS leaves me flat because there's just soooooooooo much more to music than "speed". Despite the number of notes he tries to jam into a phrase, his playing is quite lifeless (to me). There are other fast-fingered folk out there such as Stevie Vai and Al DiMeola that can play just as fast, but put sooooooooo much more into their music. Then there's others still who can say just as much as with their music as Yngwie (if not CONSIDERABLY more) with just a single well placed note....Carlos Santana comes to mind.

With that said, to me whether or not someone is "good" comes down to 2 things really...how well a given artist is accepted by his/her audience and how comfortable a given artist is with their own playing...both of which are also equally subjective. In the case of audience acceptance, Frank Zappa once said something along the lines of "I play for myself. If other folks dig it, great! If not, f_ck em". Personally I've heard some well established artists (particularly in the genre of Opera) that just scare the begeebbers out of me! LOL!!! But then there's A LOT of people who have made music that simply moves me so emotionally and so deeply that it goes beyond words.

To take those two issues to a more personal level, the best example I can think of is the gig that my band just played this last Saturday. While the bar was pretty packed, it was a VERY small club to say the least. So right off the bat there was a level of being "uncomfortable"...our front man whacked me with the headstock of his guitar at least once during that first set because we were standing so close together and a couple of people dancing in front of us knocked over my mic! LOL!!! Aside from this, while I -have- been practicing my butt off lately, because of some family issues (my father recently passed away), I found myself playing A LOT of blues instead of working on the material for the band...so yea...that first set I made a lot of mistakes! H_ll...I even started one tune in the wrong key! LOL! So between sets, I snuck out the back door of the bar to have a much needed cigarette break. As I was standing in the snow, wallowing in my own self-pity, thinking just how much I sucked, I also had a bunch of people giving me the proverbial pat on the back telling me how great I am as a guitar player and how awesome the band sounded, etc., and when I walked back in, I had two drinks waiting for me! From my perspective of my own playing and my own knowledge of my own abilities, I -sucked-. From the crowd's perspective however, the band sounded great, they were having a good time and everyone was happy! In fact the majority of the crowd stayed for the whole show (a rarity at club gigs) and we even did our first encor...the crowd just wouldn't let us leave without doing one more! So...does that make me "good"? Kinda depends on who you talk to.....

Here's the thing...with the exception of those folks who have truly bloated egos (and there's a few of them out there), a great many of us tend to be very critical of our own playing. I've been playing myself for over 30 years now, both on stage and in the studio and I -know- I'm reasonably competent as a guitar player. That said, I also -know- I could always be better and as the person playing, I always hear every last mistake I make. Often they're mistakes that even the other guys in the band NEVER hear...but -I- hear them. I know my playing isn't perfect...and it never will be. As an "entertainer" however, that's not the only thing that I need to remember! When I'm up on stage, I'm not there to please my own sense of ego, I'm there to please the crowd..and if the other guys in the band aren't hearing (most of) my mistakes, you can bet that 99% of the folks in the audience aren't either. So from that perspective, honestly...if the crowd is happy and the club owner is happy...and I get payed...I can live with my own short comings of being "only human".

From the perspective of listeners and music lovers, we ALL have our own individual unique tastes. I know a couple of guys who play death metal and when I stand there listening to them....gads...I feel like my father..."Turn that da_m noise down!!!", LOL!!! I can't honestly say that I've ever heard ANY of it that sounds "good"...to -my- ears. Equally when my wife and I first met, she was into heavier stuff as well and would just freak when I would crank Beethoven's 9th and such...and in Ludwig's day, MANY people thought he was just awful! Personal taste is really a very subjective thing.

So from that perspective I would again reiterate Ellington's words, "If it sounds good, it IS good". Doesn't really need to be any more complicated than that.

Peace,
Jim

6 paragraphs + this statement.

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Hey...never said I wasn't long winded. Personally I blame it on folks like Asimov and Dickens and such. What can I say...I learned from the best

And we know he dislikes Yngwie .....

I'm really not sure why Malmsteen draws such a pro / con response, there just doesn't seem to be any middle ground, I do find it unfortunate that much of it is based on assumptions.
I myself have never met him, sometimes have read some of the interviews and I must admit when I'm annoyed with someone's probing I have replied with a less than ideal verbal response so he seems to have a short fuse... So what ....

That being said, I grew up listening to Classical Music only, Rock was definitely not allowed in my household until after 11 years of age .. For that I am actually quite grateful as it introduced me to forms of music that I may not have had the opportunity to explore on my own....
I can understand where Malmsteen and other's are coming from as far as notes and their structures. It is not at all Blues Based, which itself is a format evolved from vocal compositions, passed down thru generations without the aid of sheet music...
I love B.B. KIng, John Lee Hooker, Muddy Waters and a myriad of other more contemporary artists. But at the same time I truly enjoy the music of John Dowland, Nicolo Paginini, The 3 Bach's, Carlos Sainz, Saint Saens, Hildegard Von Bigen ( 1078-1179 ) a bit before electricity..
Mozart, Vivaldi, Prokofiev, Tchaikovsky and multiple other forms.

Paganini was a virtuoso violinist as well as a guitarists, believed at the time to have made a pact with the dark side.... How else would anyone explain such talent .... So rampant were the superstitions that his corpse was left in its state for three weeks before finally being removed... ( in southern France that is not a good thing )
Beethoven was often regarded as insane, Bach was often at odds with the royalty. Chopin was a tragic tale, it's often the story that prodigies, virtuosos ( and by those terms I really infer to those that were/are far beyond their peers at a young age ) not the 10,000 hours spent but the one's that are just naturally wired that way. These defy Good or Competent simply on the fact that they are on a very separate functional plane than the rest of us.

Does Vai's playing render him bad or pointless There are a lot of notes and I enjoy some of it, some of it leaves me cold. By the same token I went to see Ronnie Earle at The Bull Run a while back and I wanted to run out of the place screaming. To me that is how awful I felt the performance was.. ( couldn't leave as my better half knows him well and the other members of ROB and I went as a guest ) But I try to go hear Duke Robillard as often as I can and many other's in the region such as Neil Vitullo ( Young Neil and the Vipers, The Schemers now mostly Mark Cutler and The men of Great Courage... )
To me those are sometimes great musical nights sometimes not as great, some will say Earle is the greatest.... I disagree but that is subjective. As a Kid My mom used to drive me nuts with Janis Joplin and I in turn enraged her with Savoy Brown....

Oh Well ... to each their own as they say .....

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Good for what (or who) would be my question. The previous posters have covered this nicely with concrete and abstract examples. So here's mine: my dad never considered me to be very good on violin because I mostly only played classical. The school and other awards might be nice, he'd say, but you're never gonna put food on the table playing classical violin. I eventually started playing rock guitar and my dad, despite thinking rock music was pure garbage that should be outlawed all of a sudden thought I might be good.

Why ... 'cause people were paying me (he couldn't for the life of him figure out why, but they were). Food on table, table in my own house not his ... my own heat and light bills. So, I must be a good guitarist even though he couldn't understand why. Yes, he understands technical proficiency and nowadays, he enjoys it when I play a big band era tune he remembers from his days playing horn in a dance band.

But in terms of good, his (and many of his = Depression era kids) definition is: how much food does it put on the table? Maybe my generation saw things more in terms of abstract skill or effort but to his generation, you went with what paid and what paid the most was what you were "good" at. In my opinion, I'm not much of a rock guitarist at all ... blues, jazz with a bit of classical tossed in is my style. But to my dad, while I "can" play "good" (his definition) music, what I was by far the best at was rock. And that's my point: it's purely a matter of definition.