I'm not going to cry that the sky is falling or anything, but based on trailers/things I've read, I feel safe expressing my current trepidations:
-Stark creating drones after seeming to learn it's already a bad idea in IM3 (no "man behind the armor" leading to things like JARVIS's auto-targeting; him learning that the suits are essentially nothing without him, etc). This makes me feel there can be some regression in character that would be odd to see after a whole movie exploring the idea.
-Cap getting nerfed again - mostly in power. There is yet to be any evidence to support this, but it's still on the table that they shot lots of action scenes before seeing TWS's action cut together and realizing how far they can push him while still putting him through the grinder. I was going to also say I'm hoping he comes off a bit less like a weenie when squaring off with Stark, but the clip shown definitely appears to correct that. It's good that they know that sometimes it's actually cool to be good, too.
-Black Widow backpedaling her sense of humor & TWS character breakthroughs. Obviously at this point it's clear I love TWS, and one of the things that I dug was how they rounded her out and made her... well, fun to watch. A1 was a massive improvement over IM2 in making her into an actual
character, but it was still a familiar archetype that didn't feel very authentic, and as a result, not as engaging to watch, despite some fun scenes with her (Loki interrogation, for example). Giving her a slick sense of humor that fits her personality worked wonders and should continue. I just worry we'll go back to the occasional "tough girl snarks", rather than TWS's more laid-back approach.
-Bruce losing control over Hulk all over again. I understand that he can never
truly control it, since when it snaps on its own (Helicarrier), it's pissed and unpredictable, and when he unleashes it willingly, it's the "Good Hulk", but if we see him go back to TIH stages of lying around in shambles every Hulk-out, it'll feel like we've just stripped the fun out of the character that was finally instilled in A1. So I'm hoping for deeper psychological shenanigans, rather than some convoluted silliness to make an excuse for a Hulkbuster scene.
So there goes my main concerns -- all about character regressions I guess. Ignoring Phase 2 developments, as well as A1 ones, would be the major sin in my eyes. Here are the rest:
-Black Widow and Banner dating. It's basically all but confirmed at this point, and... this is a really, really odd pairing to me. Nothing I've seen between the characters has suggested the seeds of a potential friendship, let alone romance. Even just knowing the characters on their own based on their personalities; a relationship doesn't ring true to me. It almost feels like a Mary Sue move, like "I relate to Banner -- he should totally get the hot chick, even though we're really adversarial and awkward with each other!" I guess if anyone can pull it off, though, Whedon could.
-Ultron Drones. I don't think I need to say much else. Everyone's hoping for not Chitauri 2.0 (or Hammer Drones 2.0, or Sakaarans 2.0, or Extremis soldiers 2.0, or HYDRA 2.0, etc.) cannon fodder situation, but I think it's safe to say it's coming. The main saving grace of this one could be that Ultron lives within each body, therefore forcing each character to essentially be fighting Ultron himself, which is an interesting take on it.
-Trying to widen the scope despite previous claims. Whedon's stock answer for interview questions regarding the sequel's scope was always "I wouldn't go bigger. I'd make it smaller, more personal." I thought this was really cool and fresh way to approach a sequel to a major blockbuster. But now we have this:
“It is bigger,” admits Whedon. “The cast is bigger, the scope is bigger. We have more to work with. Not that we're trying to spend more--in fact, we're trying to avoid bloat wherever possible. But with this [movie], we're on a broader canvas. We're in more countries. We have a bigger world to work with, and a bigger world for them just to be in. Once they exist as a team, we have to deal with what everybody thinks about that, and what that means to the world. It's not as simple as it was.”
This isn't necessarily a bad thing, and it's very common to have things change in movies from even an almost-final draft to a shooting draft, but it makes me wonder whether they'll be riding that line of overstuffing things. Whedon admitted to it being a challenge working the twins into the movie with Ultron being the main story, but we've also got Wakanda with Ulysses Klaw and [BLACKOUT]Heimdall and Loki[/BLACKOUT] and all these other things in there.
These are just some observations that have given me pause. I hope I'm wrong about them of course, and I'm sure I will be for at least most, but just some things that have come to mind based on the marketing and interviews released thus far.