I don't know about a Wolverine/Cap connection. I do remember a story from the 80s or very early 90s about Nick Fury and Natasha both using a serum that was similar to Cap's serum, and needing to take a new dose every couple decades.
Anyway...
I gave it an A+, not meaning that it was perfect; I get an A+ with anything over 90%.
One thing that bugged me a few scenes were the Glocks used by SHIELD agents, particular scenes were Fury about to try to shoot down a fighter jet with a handgun, and anytime Natasha was shooting at anything. In the comics SHIELD always used some kind of lightweight blaster and you could imagine it being as powerful as the plot required. It was just too unlikely that Natasha was shooting Chitari down with a 9mm, these troops were supposed to be taking over the friggin planet, you should need more than a handgun. Something at least bigger looking with a bang like a .44 magnum, leave it to the viewer to imagine how much more powerful it was than a typical cop's gun.
The other thing was the filming of Cap's moves, it was much better done in his own movie, the CGI/cinematography in Avengers didn't convey his abilities nearly as well. Thinking of the scene where he is chasing down the spy just after Erskine was killed for example, it wasn't just the muscles, his moves really conveyed the idea of a superhero. In Avengers it came across a lot more blandly.
Thor need a better "moment", he was a generic Thor, compared to the other characterizations. Not that he needed another 15 minutes or anything, but just a couple of shots that gave him some feelings. Like keep the Jane Foster situation the same, but show him gazing silently at her picture for an extra second, let us imagine what he is thinking, and then move on.
The scenes in the beginning with Loki appearing and stealing the Cube were weak. In theory, SHIELD protected the installation from assault from outside, and the chaotic breakout from with should have been unplanned for, that's why it should have worked. That makes sense, it just wasn't really shown that well. The car chase was overdone, and if you were going to do it, give it more tension and suspense. Like the copter gets hit, Fury jumps, it was over too fast without really feeling that Fury was ever in that much danger. We needed to see and feel his stress in that moment.
Meanwhile, the way the end fight flowed from battle to battle without cuts, but just panning from one hero to the next was inspired, I loved that. I loved the Hulk and every scene Ruffalo was in, I won't repeat everything everyone else wrote, but I too want more Hulk. I'm not actually sure it would come across the same way in solo movie, because a lot of the best scenes were Banner playing against Stark, or Hulk playing against the other heroes, hopefully there would be supporting characters in a Hulk film that give Ruffalo someone to match against in the same ways.
Downey stole every scene that Ruffalo didn't, his encounter with Loki in the Stark tower was beautifully set up and executed. Loki/Hiddleston was superb throughout and will be hard to follow. The unspoken backstory with Widow and Hawkeye has left me wanting THAT movie.
I like the idea above of Coulson becoming Vision. Or being the first LMD. Which reminds me, I knew who the real fans were in the audience, the ones that got Tony's LMD reference.
For some reason I was thinking of the Ultimate Wasp which meant a little more Asian. I guess they could go anyway they wanted be it 616 or Ultimate.