1. The writing was bad. It was predictable and formulaic, as if it were written by committee. Everything happened right when it should, and went right according to plan. The characterizations and character evolution weren't natural, but they happened because it was time for them to. Relatedly, their most talented actors were underused.

2. Yondu was distractingly bad. Michael Rooker should not have been let near that part. His southern drawl (why?) and Merle-like delivery broke up the movie every time he was on camera, and he had no arc, or reason for me to care about him at all.

3. The ending was terrible. It didn't resolve some things (Ronan's impressive hammer went unused, Nebula just sort of went away), and deus ex machina'ed others (why Quill wasn't vaporized when he touched the gem, Groot having whatever power they needed at the time). It was very unfulfilling.

4. All of the best parts were in the trailers.

It was a gorgeous movie, imaginative and compelling. The visuals were great - I loved the look of the Kree ship, and of Thanos, who looked and sounded perfect. There were a few moments of surprising vulnerability and only one character (mentioned above) was a real paperweight. And the post-credit scene was hilarious.