Honestly, the Crystal Corner is about the best example of a neighborhood bar I can think of. Great taps, comfortable feel and quick and competent bartenders are the norm here.
I can still remember the first time I went - my friend and I had just turned 21 and we were exploring the non-State Street parts of Madison for the first time. My buddy's father must have heard the same things about the Crystal because he told us to "never go to that place - people get killed there!" So of course we went and the warnings of imminent demise proved incorrect. For a neighborhood bar the Crystal Corner is remarkably friendly which for me is the be-all end-all. I've always found the insular nature of some places off-putting - the "swivel-factor" of regulars looking you over when you walk in is an immediate strike against for me. That's why the Crystal Corner is the best, because the atmosphere here is the perfect mix of casual, friendly and fun.
If it were just a comfortable place to hang out the Crystal would still be one of my favorites in Madison. But the reason I think the Crystal is a five star joint is because of the great live music. I've gone to shows here both large and small - sometimes the place has been so packed that I've watched the band from across the bar. Most shows tend to be a bit more low-key, with plenty of room and good sightlines. I wouldn't have ever thought it, but looking back on the last five years I've got to say a couple of my favorite shows have been at the Crystal. And last (but certainly not least), this place is frickin' famous - Okkervil River namechecks the Crystal Corner in "A Girl In Port". So they've got that going for them, which is nice.