The drag-and-drop admin stuff that's being developed is superb. But as people keep saying, WordPress integration is the biggest (or one of the biggest) desires.
Regarding WordPress, the top page of the zenphoto site says it best: "Major plans include native WordPress integration (in many sexy ways)..."
I'm not sure what "native" means, but it sounds good. And of course "sexy" sounds good (it probably means "AJAX-y").
In my view, full "native" WordPress integration would mean a few things:
1) Integrated search. You search WP blog postings and pages (and user comments?), AND ZP album/picture titles and descriptions (and user comments?). Plogger and Gallery 2 do NOT have this. Gallery 2 is considering it; Plogger apparently is not (too much work, a dev posted).
2) Ability to make the ZP gallery the default page of the site. For photographers, artists, etc. doing portfolio sites who want thumbnail portrayal of their work right on top. Maybe this is done by defining a WP page that links to the ZP gallery, then using the "Static Front Page" plug-in that lets you set a page as the WP default. Or maybe there's a ZP "sexier" way (I'm all for it). This much the K2 ZP theme would presumably allow today, but that theme seems to have a significant bug in it at the moment (see the thread in this forum).
3) Easy, intuitive insertion of ZP content into posts and pages. Ideally, this will involve an interface for non-tech users (i.e., not requiring copying and pasting URLs from different browser windows, etc.). Sure, insertion of random images etc. into sidebars is great, too. But if you're writing about your galleries in any way, joining text and photo content easily by a layperson is essential. Some kind of integrated posting / gallery browsing that allowed drag-drop of thumbs into text would be beautiful.
Right now I'm using the Gallery 2 plug-in (which deserves lots of praise, including for the docs), but which frustrates me for two reasons (at least): you have to paste a URL to do it (bad workflow and too technical), and you can insert only the scaled-down image size that you also use for your sidebar photos. Makes for a very boring layout. I'd want to see image size variety.