While some of the characters were a surprise, some were pretty obvious.  There are also no outlandish characters from the later games in the series that show up thankfully, because I would have exploded if I saw a Cyrax or anything else from Mortal Kombat 3 and on.  Also, the DC characters break down pretty even between villians and heroes.

Thanks to 1up.com for the list

Mortal Kombat Universe:

• Scorpion
• Sub-Zero
• Sonya
• Jax
• Shang Tsung
• Liu Kang
• Raiden
• Kitana
• Kano
• Baraka
• Shao Kahn

DC Universe:

• Batman
• Superman
• Catwoman
• Green Lantern
• The Joker
• Shazam (We have no idea why they aren’t calling Captain Marvel by his actual name)
• The Flash
• Wonder Woman
• Deathstroke
• Lex Luthor
• Darkseid

Evil Avatar had the explaination for the Shazam vs. Captain Marvel issue, stating that Marvel Comics now owns the trademark to the Captain Marvel name.  While DC can use hte name in the comic books, they cannot use it in any other promotional items outside of the comics, hence the Shazam name in the game.