If there's one thing I'd single out about Twitter for Mac, it's the non-standard window implantation. The titlebar at the top is invisible and tiny, the plus button is more useless then a normal OS X app, you can't drag the window off-screen, and if your configured displays are changed and you suddenly have less vertical resolution there's no way to resize the window.[1]
I could keep bitching about things (no font size adjustment, bizarre animations that give no context, poor highlighting of selected tweet), but there's no real point - if you like it, you like it, if you don't, you don't. :-)
[1] This was all in version 1.0 - perhaps they've changed/fixed some of this, but I haven't felt like going back and finding out.
