- no xterm mouse support (no, option-click is not the same)
- the utterly stupid default for PageUp/PageDown/Home/End, which is correct for OSX UI, but utterly wrong for terminals.
Less obvious (relavent to UI):
- No notification support
- No window splitting (yes, screen works, but, re: changing panes, see xterm mouse, above)
- inability to launch a particular named window group from the command line (if this is possible, please tell me how! :-))
...And much, much more! [1]
[1] :-) And I guess that's about it for the things that bugged me compared to, you know, the actual xterm.