Hey, It's That Sound!: In the 360/PS3 version of the second game, when The Thing punches Lady Deathstrike away, the sound she makes is the exact same sound Elektra would make in the first game when she gets hurt.
Steve Blum is once again Wolverine and Venom in the first game. David Kaye is Captain America and Nick Fury in the second game. In the first game, Cam Clarke was Thor and Daredevil. In the second game, Jim Cummings takes on the role of Thor.
Luke Cage was voiced by Greag Eagles (Grim) in the first game and Khary Payton in the second game. The latter was also Blade in both games.
Robin Atkin Downes voices Crimson Dynamo, a non-hulked Bruce Banner, and Dark Cyclops (a sort of reprisal, as he voiced regular Cyclops in the first X-Men Legends) in the first game. In the second, he voices Cable for both the campaign and the DLC.
Arin Hanson, or EGORAPTOR, voices Bruce Banner in the PS3/Wii versions.
Ymir in the first game and the President of the United States in the second game are voiced by Paul Eiding.
Ben Diskin voices Spider-Man and Penance in MUA 2. The first one is especially ironic since Ben also voiced another Venom.
The Other Darrin: A lot of the voice cast changed between MUA 1 and MUA 2. Pretty much the only notable cast members not to change were Steve Blum as Wolverine, John Kassir as Deadpool, April Stewart as Ms. Marvel, Dawnn Lewis as Storm, Nika Futterman as the Black Widow, and Danica McKellar as the Invisible Woman.
The first game's DLC was only released on the Xbox 360, thus screwing PS3 owners out of characters like Cyclops, Hawkeye, Magneto, Dr. Doom, Venom, and Nightcrawler. Oh, and the Hulk (the one character everyone wanted in the game but wasn't) was in the DLC too, by the way.
The second game's DLC was released, and then taken down after several weeks. For six months. It wouldn't be so bad, except a) The DLC was released over month later on the PS3, and the DLC was taken down at the same time on Xbox Live & PSN; b) the DLC was taken down before Christmas, so anyone who was getting the game as a present couldn't get the extra characters; and c) Activision had the audacity to claim the DLC was a limited offer - It wasn't, they just hadn't renewed their licence with Marvel & the DLC had to be taken down. Hell, people who worked on the game complained about this one.
The DLC was eventually made available again in June 2010. Several months after people stopped caring about the game. Unfortunately, this doesn't apply (yet) to PS3 users outside the U.S., and there's no word on a release date from anyone.
And on January 1, 2011 the DLC vanished again, permanently.
The second game's DLC went back on the PS Store once again in January 2012, until January 2014 when most of Activision's Marvel games were delisted due to them losing the rights to make Marvel videogames. Only the tie-in game for The Amazing Spiderman was spared.
Concept art from the first game indicates that in one level, Captain America would take his friends back in time to World War II, and they would aid the allies on the beach of Normandy. Talk about Best Level Ever that never was.
Electro is present in one of the villain concept art pieces in 'MUA1, but was not in the game. Guess who shows up early in MUA2''?
Other planned locations in the first installment was the Baxter Building and the X-Mansion. They both got axed.
This applies as a whole to people who've just learnt about the series on the consoles and will now never get the DLC (See above for details). For PC Gamers it's better, as they can get the characters for the first game at least through mods.