What I really like to see:

Sequence number
If it came with sketch
If it came with MATCHING background
(I think you should state it if you posted a cel
with an added/wrong background too)
Why you like it
Humorous little captions

What I don't like:

Where the cel was purchased
Screen captions

I ALWAYS read the funny little things people say 
about their cels or why they like them.  I like
to get to know the collector.  I don't need a 
synopsis of the scene or summation of the anime.

Chances are if I am checking out the cels, I have
already seen that show.  Personal comments beat
facts, except the technical info on the cel 
(sequence number, etc.)

Anyway, I think if people are taking the time to
visit a collection, they will probably take the
time to see what you said, as long as it is to the
point.

Personally, I am not a big fan of those gallery
that just have the scans and no info.  I like to
think I am visiting a reflection of the owner, 
not just their art collection.