Here's hoping that you do decide to make a gallery
-- I always love browsing other folks' cels!

Let's see, now...

1. Page Write ups

I used to have these, before I changed my site
layout.  I'm thinking of putting them back -- I
guess the answers to your question here will help
me decide as well.  *grin*  Personally, I like
reading them, but not many people have 'em on
their sites.

2. Specific cel write-ups

I always, ALWAYS read the commentary.  :)  To me,
that's every bit as interesting as the cel image
itself.

3. Watermarking cels

Some folks watermark their cels to prevent image
theft -- not necessarily from folks who want to
use an image on their wishlist, but from the rare
worthless bastard who thinks it would be nice to
post somebody else's cel scan on eBay and auction
it as if it was their own.

Personally, I don't watermark my cels.  The only
reason I post them is because I want to let
everyone else appreciate their beauty, and a
watermark would mar that beauty.  But in doing so,
I know I'm leaving myself open for the risk of
image theft.

There's no easy answer to this one.

4. Categorizing

I'm a big fan of galleries organized by series,
although I do have global sections on my own site
for shows which only have one or two cels, for
genga from digitally colored shows, and for my
assortement of nekkid man cels.  *grin*  I'd say
it's a judgement call -- if a non-series category
makes enough sense that people know exactly what
they're going to find there, it might be every bit
as helpful as a series category page.

5. Anything else that people really like/dislike 
about galleries to help guide someone making 
their first one?

Yep -- page loading time.  The more images you put
on a page, the longer the page takes to load, and
if it's slow for me to see on a cable modem, I
know that dial-up users are envying the dead.  ;)

In addition, I like a clean format, without too
much graphical clutter, animated .gifs, and
elaborate graphical backgrounds to steal away the
impact of the actual cels.

Just my two cents' worth, of course.  :)

Best of luck in putting together your gallery, and
I really look forward to seeing it!!