Short, simple, catchy lyrics and melodies. Good performances. The characters of the band are good (personality, style, trend setting...etc.).
If you want to REALLY get serious, we can pick apart slash's playing note for note, and we don't even have to talk about music theory or ANY of that bullshit.
It's okay to like a guitar player even if he isn't the best in the world. It's okay even if he's not good at all. People love Neil Young..... he's not good at guitar, lmao.
In reality, the best guitarists are probably completely unknown to the world. Gypsy kids raised in spain who are given a guitar when they are 3, and that's all they do for the rest of their lives. Probably the best players in the world. You really think some guy in a top hat with his guitar down past his knees playing pentatonic scales over one chord jams can out-do them? No. You can still like slash more, though. That's freedom of taste and expression, and I support that.