One word:

AWESOME!

(More words, details, and feelings down lower, where I can hide the spoilers.)

*SPOILERS*

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It was pretty much everything a superhero team-up movie should be.  Full of action and amazingly badass moments for everyone, great character moments and interaction.  Pretty much everyone had their own individual mini-character arc.

It was funny.

It was dramatic.

It was even sad on a couple occasions.  Agent Coulson bit it.  Did not see that one coming.  I didn't cry but DAMN, heartstrings.  They were quite yanked on.  Especially since Coulson got some great little cute moments where he was a total Captain America fanboy.  HE HAD TRADING CARDS.  THAT HE WANTED CAP TO SIGN.  TOO PRECIOUS.

And the pain in Thor and Loki's voices as Thor begs Loki to come home and Loki says he doesn't have one?  Ugh, ouch that one got me right in the sibling fluff love circuit.

I just... I adored the character interaction in this movie.  Pretty much everyone got a good bonding moment with everyone else.  Cap and Tony bickered and didn't quite see eye to eye at first but eventually dropped their differences and teamed up to fix the helicarrier's engine.  Bruce and Tony had mutual respect for each other's great intellects.  Cap and Natasha both treated Bruce like a person, instead of a monster.  It was heartwarming and awesome.

Um... FAVORITE MOMENTS!  Yeah, let's do that.

1. HULK INTERRUPTING LOKI'S HANNIBAL LECTURE ABOUT HOW, "I'M A GOD, YOU'RE PATHETIC BLAH BLAH BLAH" TO JUST TOSS HIM AROUND LIKE A RAG DOLL AND THOROUGHLY CURBSTOMP HIM.  My theater burst into both laughter and applause.
2. Natasha putting Coulson on hold while she demonstrates she actually has her "interrogation" completely under control.
3. Anything Natasha did really.  'specially psyching out Loki.  Yeah.  She outacted the God of Deceit and Trickery.
4. Hawkeye shishkebabed a hovercraft behind him with an arrow without even looking at it.
5. Oh yeah, and then he shoots an arrow at Loki.  Who catches it in midair.  Oh and then it goes BOOM.
6. Can I say everything from the final battle actually?  Seems like it would be simpler.
7. TONY/PEPPER BANTER AASLFKAJHSFLAKSF MY SHIIIIIIIP!
8. Jane is safe.  And Thor is so happy about that.  My shipper feelings explode.
9. Hawkeye and Black Widow need to make out.  Hot damn they make an epic Battle Couple.
10. How did my list of moments suddenly turn into bullet points of my shipper feels?
11. CAP'S BIG DAMN HEROES ENTRANCE.  FOLLOWED BY IRON MAN'S BIG DAMN HEROES ENTRANCE.
12. THOR BRINGING HIS HAMMER DOWN ON CAP'S SHIELD AND THINGS GO BOOM.  Cap's not even scratched of course.  But everything in a five mile radius is kaput.
13. Seriously.  This movie is awesome.
14. There is no point 14.

And the Stinger after the credits reveals the Bigger Bad.  Who is... some guy I can only assume is Marvel's equivalent of Darkseid.  Looking up this information I found out his name is Thanos.  This information will obviously have more relevance to Marvel fans than me.  (Sorry.  The DC girl here has little to no clue who the guy is.)  All I know is that he's scary looking and apparently REALLY REALLY BAD.

And the Stinger after the rest of the credits, with the team munching fast food?  Hilarious.

All in all, an amazing cinematic experience.  I'd say go see but uh... you probably won't be able to find a showing for days.