Here's why I think Bebop is considered so great by so many:
1) It has very few of the features that turn most people off of anime: Annoying screechy voices, sudden use of chibi character designs and weird faces to emphasize that something going on is "funny," stock gags that barely humorous to begin with, horrible English dubbing that forces you to watch using subtitles, etc.
2) It has slick, fast-paced, grooving style in which stuff actually happens, instead of filling the episodes with people preparing to do stuff, talking about how they're going to do stuff, and reacting to stuff that's been done.
3) Some of the best animation ever in a TV series.
4) It touches on a number of philisophical and sociological issues, but never dwells on them -- it leaves the dwelling up to the viewer.
5) Legitimate tension, because the characters occasionally succeed, but fain more often than not.
6) Arguably the best soundtrack for any TV show, live action or animated, ever.
7) Spike Spiegel
8) Motherf*cking Spike Spiegel
9) SPIKE SPIEGAL GODDAMN IT!
So yeah, I'm sure there are plenty of other animes with better premises, more engaging overarching storylines, and possibly even better characters, but I think it's these qualities that make Bebop so popular in the eyes of many -- especially in the eyes of people who usually can't stand Anime.
Also, I'm not saying that Bebop is the only anime that has each of these qualities. It just combines them so well.