Nothing. I love it.
I've got true Flash support (and Java, of course).
I've got my extensions for anti-adware support, ABP and NoScript. NoRedirect silently does it's thing, along with a handful of GreaseMonkey scripts. Cookies Exterminator silently erases standard and script engine tracking cookies. User Agent Switcher gets me around annoying webpages that block me. Prefbar does all sorts of weird proxy stuff and etc.
Support for a slew of protocols. Built in email client, calendar and tasks (Lightning) and ICQ chat (Chatzilla). Gmail support with gContactSync, Provider for Google Calendar (and Google Tasks), EnigMail and PGP for encryption. Of course we can't forget FireFTP, that's the straw that broke the camels back and had me writing nasty emails to the head FireFox devs. ErrorZilla Plus intercepts useless error pages and gives options like URL repairing and accessing Google cache, the Wayback Machine and etc.
I've got ALL of my favorite extensions like Session Manager, Down Them All, FlashGot, ImageZoom, Search by Image and ScrapBook I've got the original Speed Dial extension (which doesn't have adware built in). I've got my highlighter and sticky note extensions (which seem to have disappeared from Firefox for no reason I can discern.. maybe it's not possiible without WebExtension?).
I've got interface enhancements, Modern2 gives me a dark theme, and puts my browser buttons back where they belong. Flagfox and ShowIP give me visual indicators of WHERE I'm browsing. Download Status Bar gives me my download bar back. xSidebar gives me my sidebar back with a slew of custom sidebars, including ScrapBook bars, developer bars, eBook stuff, and the standard history, search and etc. sidebars. (HEY! It just occurred to me that torrents.me would make a great sidebar.

) The SeaFox extension puts the new tab button back on the right hand side (where it belongs!) and puts the tab close buttons back on each tab, along with various other enhancements. MrTech Toolkit tweaks the GUI and operational characteristics.
I have a slew of priceless developer add-ons. ColorZilla gives me an eyedropper, color picker and gradient generator. MeasureIt lets me measure dimensions. Dom Instector, Edit CSS, HTML Validator, Live HTTP Headers, all allow me to play with coding pages live. SQlite Manager lets me mess with my SQL databases. Of course, I've got the formidable built in DevTools.
Color Transform lets me automatically colorize websites that irritate my eyes after extended viewing.
What can they do better? Not much. Mousing over Amazon pictures takes a while to load, but I expect that that's some faulty script coding on Amazons side that doesn't allow the pics to pre-load. They COULD integrate various protocol controls. I never understood why the Mozilla broswers never adopted (Thunderbird), Lightning, Chatzilla, FireFTP and intelligent error page handling (ErrorZilla). They need to fork off Firefox, definitely. Put the GUI stuff like the new tab button and browser buttons back where they belong.
Basically the only thing I would like to see them do is improve performance and simplify the codebase for SeaMonkey. If they're going to ADD anything, add in sensible features.. free of adware and etc, of course. Oh, I would LOVE to know why recent versions of browsers all suck up like 500MB of RAM. I'd like to see WTH ever causes that reverted.