1) Simplicity of design.  While it might be fun to 
play with fifteen different fonts, twenty banners, 
streaming audio, and a montage background, this 
really tends to distract from the content you're 
trying to feature.  If your goal is to display 
your cels, they should be the focal point of the 
site.

2) Captions!  ^_^  What can I say, I'm a sucker 
for 'em.  I love to see cel galleries that include 
either information about each character or scene, 
some fascinating tidbits about a cel itself, how 
you acquired it, or why it's important to you.

Just my two woolongs' worth, which in the Bebop 
universe, won't even buy you a cup of ramen.  ^_~