Unified Inbox: Along the lines of what RIM does. Have IM, e-mail, call log, etc all go into one spot. It would not replace the individual apps but just collect their data together in one spot as an alternative view. Also extensible via SDK. So for example, you might have a new voice mail, 2 missed calls, an IM, and a notification of an application update from AppStore, along with some new e-mails all in one list. Touching any item would launch the native application.
Spruced up Lock screen: Either more information in the traditional "Today" style view or something fun and gimmicky like animated wallpaper or widgets.
Copy & Paste: I don't care about the feature. I probably wouldn't use it. I just want people to shut the fuck up about it.
MMS: Same deal. I've used MMS maybe once in my life. I don't talk to people who can't send and receive e-mail. It would simply shut people up and that would make me happy.
More UI customization: Just something as simple as color themes would be welcomed.
Better App Management: Something along the lines of Smart Folders by category and also a "most played"
Voice Over: Read the from/subject of a new e-mail, the text of an SMS, etc. Definitely an option, not a default. (lots of potential for misuse, annoyance and embarrassment) It'd be great for driving. Accessible to third party apps via API of course.
Widgets: Something you can place on your home screens. It'd be slick to have a page devoted to just random information without having to launch an app to find it. I would suggest with Widgets they run in a zoom-ed out mode (resizable though) and when you touch them they zoom in and/or display more information. Very light weight, very quick to switch between them