Really? I didn't have a problem with the voice acting myself. Other than Black Widow. That accent. Just ugh. I had issues with some of the casting choices (Cap sounded way too "Golly, gee" for me. There should have been a hint of that, but Ultimate Cap is still very much a soldier) but didn't have a problem with the delivery beyond Widow's.
The plot was kind of pointless, though. They took hints from the Ultimates comic book, but it didn't really go anywhere. "These aliens have been on the planet since WWII, where they helped the Nazis. Since then, they've stayed on the planet for some unknown purpose, then blew up our tracking satellite. Now they're attacking us. Yay, we blew them up."
Still, it had some great one-liners. Thor, and everyone's reaction to him, cracked me up, and the final fight with the Hulk was pretty cool.
(Edited to fix annoying typo)