Ive tried all the native mac twitter clients out there (that I can find) and so far Tweetie is the best (Nambu would be if it were not so unstable). A few features would make tweetie stand out and I would happily shell out for a full version (and even keep the ads on!):

a) Groups. The UI is perfectly setup for groups already with the left hand column. I like to put my "A list" of must reads on its own screen so I am not distracted by all the people I only care to read occasionally.

b) Remove all. I would like to remove all tweets from the current window so the next time I open the app I know everything in the window is new.

c) Inline tweeting. The separate window for composing tweets takes me out of the flow of the application. An inline composer would be nice.

d) Auto complete for @names.

Other than that great job!