etags -- it's just an nightmare, full stop. It only gets you where you want to go half the time at most. And then because the GUI is so primitive you can't do basic things like click on a symbol and have it jump you to the definition, you're stuck using the keyboard interface designed for work in a terminal.
An etags that actually worked properly is one of the most basic things that could improve my productivity monumentally.
CEDET -- last used about 5 years ago, gave up because on larger files/projects it would just hang my editor for a few seconds as it processed.
Variable/symbol completion -- see etags.
I'm not doing a very good job communicating my dislikes / pain points because I'm not very good at communicating. Please do not respond with a "but these aren't problems because..." because they are problems for me, I just do a poor job of communicating.