Well, definitely have to mention iPistol and iCruiser (or iBattlecruiser, or iBattleship, I'm not sure). The iCruiser is on a bilboard in the first mission.
Then there's the Terrible, Terrible damage, that even appears in the challenge missions if you kill a lot of at the same time using the High Templar storms.
And of course, Grunty The Murloc Marine.
One of the things that really made me giggle madly was in the lost viking arcade. "HE DOES NOT LIKE YOU!!!" *in flashing red letters*
Oh, I can't believe I forgot the "It's a trap!".