The question is beckoning for very relative answers. There's no universal definition that characterizes a 'good guitarist'. However, there are a number of criteria.
There's the guitarists that are good at 'composing' music, and they generally have a good 'feel' for the instrument, being able to say use a pleasant line of notes when improvising as opposed to running up and down harmonic minor arpeggios all over the various octaves of the key they're in. I think Mikael falls under this category, as he doesn't seem to be a technical player, he goes for feel, and that is very much expressed in his music.
There is the technical guitarists, who pride themselves on their ability to shred very fast using quite simple ideas, and usually lack alot of feeling and their playing is thusly reflected as largely formulaic. I would say Alexi Laiho and John Petrucci are guitarists that fit under this category, as this style is very much reflected in the music they write.
Now here is where we really start getting opinionated. When you talk about the average 'ameteur' or 'experienced' guitarist I suppose there's a number of things one judges them on. One would be their knowledge of musical theory, for which I don't have a very big fondness of myself, although I do understand that it is necessary to a degree in helping me write music and improvise (although im too afraid to touch it out of fear for turning into a boring scale-run player). There is the ability to read sheet music, which I believe would be completely unecessary for the average metal player. Going by ear or tablature suits fine enough when playing this realm of music. There is the ability to play 'fast' which usually coincides with knowledge of scales, usually consisiting of someone being able to run up and down them, using certain notes, changing key and whatnot which gives you improvisation. This ability is only necessary based on the guitarist's own prefference. If said guitarist is not a lead player, or doesn't wish to be, there's really no point. There is the ability to accent notes correctly, one thing new guitarists have a large problem with doing and make a song that they play, sound in fact, like the song that they're trying to play. I feel this is a very important aspect of guitaring... for its not an instrument where one just frets and plucks, there is a certain degree of 'feel' and precision that needs to go into it.
I'm quite sure there's more but I've lost my train of thought, and I hope that gives you an outline of what I believe is a good guitarist.