Keyboard shortcuts... really nice I use all the time feature:
ctrl C for copy, ctrl V for paste, ctrl right arrow jump till end and left for jump to margin... ctrl+shift+ one of the arrows jumps one word and selects, shift+ctrl+p for screenshot... etc.

easily choose and copy a text a friend wrote and paste it in to your own text.

the merge contacts... really easy to merge and select common name.

Calender is really growing on me with all the possibilities of making multiple calendars in different colors... I have NEVER been this organized

Multitasking goodness in general... so easy to jump from one thing to another... looking something up on the net while chatting with a buddy and listening to some music.

UR-QUAN MASTERS!!! need I say more :-D

oh and all the little things you take for granted:
f.ex. led flashing when message recieved, missed call etc.
search contacts just by starting to type name on keyboard.
Multiple desktops. (with different or loopable wallpaper)
integrated Skype.

last but not least: the endless possibilities of open source.