If you're seasoned:
Uncheck Enable Simple Command Card - Will bring back the good ol' command card with partrol, hold position, and all the other classic commands.
Uncheck Select All Larvae - Will allow you to click on a single Zerg larva, should you choose.
Check Enable Enemy Unit Selection - Will allow you to select enemy units and structures.
If you're a purist:
Uncheck Enable Starting Worker Rally - Will allow you to assign your workers manually at the start of a match, just like in previous versions of the game.
Uncheck Display Experience Points - Will stop the XP points from popping up over kills and such while you're in a match. (XP is still earned.)
Uncheck Always Show Worker Status - Will only display worker counters on "town halls" and Vespene Gas structures when cursor is hovering over it.
Enjoy the new UI of Starcraft II!