Most of these suggestions are taken from Visual Assist for Visual Studio. It's an amazing add-on, and including most of its functionality into TheIDE would be huge. The visual assist website very clearly documents most of its extensions, and so I've linked to many of them here so you can see what I mean.
Assist++
Show autocomplete on "::" in addition to "." and "->". Especially for calling static class functions and filling out member functions (if for some reason I forgot to alt-c).
Show non-exact case matches in completion window. Don't remove matches that don't match the exact case, but just list exact-case matches first and non-exact case matches next.
Insert the top match if the user hits TAB. (Saves having to hit down arrow and enter, which make the hand move far.)
Improve "Enclose selected text". Allow using '/' to comment lines with // and '*' to comment lines with /* ... */. Also, if the lines are already commented, then un-comment them instead. See http://www.wholetomato.com/products/features/surround.html
Switch between cpp and h file. In Visual Assist, Alt-O is "open opposite" -- if you're in a .h file, it will open the corresponding C file and vice versa. Very convenient! See http://www.wholetomato.com/products/features/header.html