Here's my .02
Yes, I agree it is an "essential" of anime. That's why it remains my #1 on my top 10 (and probably will forever and ever, amen

).
So what makes it "essential"?
For me, it's how the series as a whole is constructed. Anyone, and I mean anyone, can watch...say...episode 5, then episode 18...and they won't be completely lost as to what's going on. Even Spike's backstory, which spans over 5 eps, is done in such a way that you don't have to see them all to understand it.
Most of the series I've seen (which, I will admit, I haven't seen dozens like many of you), require that you see them in order, or at least see more than one. Cowboy Bebop is great because one can pop in any of the DVD's, randomly pick an episode, and enjoy.
And on other notes...it has great character development, animation that still stands up to what's being produced today, and a killer soundtrack.
In short, Watanabe is my hero.
