The Pros: If you're into comic books this movie shouldn't bore you too much. Like most films based on comics, there of a number of references and in-jokes for fans (Jocasta cameos as a base computer, the Savage Land is mentioned, etc.) As usual, there's plenty of action, albeit tame action, but still enjoyable. Part of the plot even involves fighting Iron man robot versions of the original Avengers. The Hulk is sporting the old-and-grey look from Incredible Hulk: The End, which looks pretty cool, though certain shot look very cartoonish. The animation quality hasn't gone down since the other animated Marvel movies, though it isn't as spectacular on the whole. This whole story is very much in the spirit of other team super teams, such as the Teen Titans and the Power Pack, so if you were a fan of those franchises, you should enjoy this.
The Cons: Obviously this film is far tamer than other Marvel Animation works, seeing as it's their first PG-rated film (all the others were PG-13). Thus all real sense of danger is eliminated. No real violence here, just kid-friendly smash-fests and explosions. They're not even allowed to use the word "kill" or show anyone dying (except robots). Most of the dialogue is just fair to corny, and none of the jokes are really laugh-out-loud funny.
The End: In 2005, Marvel started a comic series called Young Avengers, featuring teen heroes who based their identities on the Avengers. Though that may have inspired this movie, they are nothing alike, in either story of tone. This actually has more in common with A-Next, a comic starring the Avengers' successors in an alternate future. That's not necessarily a bad thing, but personally would like to have seen the Young Avengers more. Rating this against Marvel's other animated features, this is definitely not their best. Not terrible by any stretch of the imagination, but not their best. Certainly worth renting, but I personally wont be preordering this on Amazon.
Overall, I rate Next Avengers: Heroes of Tomorrow as a 6 out of 10.