I used to be a big proponent of Mauku (which was built for Jaiku, if I recall correctly, hence some of the missing features) but coming from Gravity on Symbian, there's several things that Mauku is missing (running the Beta 4 on my N900, btw)

1. Multiple accounts. This is crucial for me. I have 4 twitter accounts that I monitor/update, and I need to be able to use all of them at the same time (to switch instantly, without having to re-enter all my credentials each time)

2. Update from the timeline - I'm confused as to why I need to back out to the main screen just to update. Why not add a 'New Post' button to the options, or better yet, allow me to simply start typing from the Overview?

3. Twitpic support would be cool.

4. A 'replies' area would be cool, too, where I could simply see a single stream of all the @replies to me.

I'll keep putting together feedback as I use it.