There are no good desktop Twitter apps out there. I seriously can't stand any of them. So this is a plea to the software developers out there: Please, make a good one. To get you started, here's a list of features I'd like to see:
Not Adobe AIR
Please, just don't bother. There's a ton, and they all suck. You know what else? They might have a chance at being decent if they weren't in AIR. It just isn't very good.
Standard OS X chrome/behaviors
I'd like it to look like a Mac app. Why Twitterific looks/behaves like an Adobe AIR (cr)app, I'll never understand.
This one's a must. Twhirl does this ok, but I think there could be better ways of dealing with it.
I'd like to be able to filter things out, or highlight especially interesting tags. Especially useful during events that I can't go to / don't care about.
Both searching the tweets you have cached, and also easy access to search.twitter.
The standard stuff
There are plenty of things that apps like Twitterific, Twhirl, and the like do well. Easy replies, easy previewing of other peoples @replies, easy linking to whichever url-shortening service i desire, an option to automatically expand the urls when I'm viewing them, etc, etc.
I know it's possible. Tweetie, for the iPhone, comes close. It handles multiple accounts well, and really, it does a lot of things very well. Too bad it only comes for the iPhone.