A few things I can think of:
* Better support for background tasks .. :mak, :grep, system(), :! without interrupting editing.
* Fancier errorlist and locationlists. cexpr()/lexpr() really make you fit round pegs into square holes.
* Better omni-completion performance (any completion dialogs other than <C-N>, buffer keyword completion, are unusably slow for my purposes).
* Built-in support for some filesystem operations. NERD_tree/:! can be clunky, and dropping into the shell isn't always ideal.
* Real shell buffer .. I know I'll get my head cut off for this because it is quite emacs-y, but it would be nice to be able to run my shell in a buffer. At the very least, it would be nice to have :sh be a somewhat capable terminal that could handle colors and generally feel like a normal terminal.
I'm excited to see 7.4 because it looks like some real new direction for the project, for better or worse. I'm sad to see VimScript de-emphasized, having invested a good bit of time in getting good with it, but VimScript is pretty slow, so this is good news.