All of my complaints are pretty minor.
1. Dave Baustista needs acting lessons badly.
2. While I loved the post-credit scene from a comedic standpoint, I wish they would've had a mid-credit scene that actually meant something to the MCU.
3. I wanted more Nebula.
4. I was hoping for an appearance by Richard Rider, but oh well.
5. I wanted the story behind the Infinity Stones to last a little longer. I felt like more information should have been given.
I thought Dave Bautista's acting was perfect for this version of Drax. Though, he probably wouldn't be the best choice for other heavy hitter characters.
I agree that it would have been nice to get a glimpse of a future Marvel film, but the whole movie was filled with teasers of where the Marvel Universe is going.
Nebula was one of the most (if not the most) interesting characters in the whole movie. Seriously, everything about her character was awesome (great casting, great design, etc.). I'm looking forward to the Guardians running into her again. Will she still be aligned to Thanos, or will she be on her own?
Richard Rider is a pretty important character, but introducing him could have complicated the plot even further. I like to look at the Nova Corp big fight as a precursor for his introduction. Quill's involvement as a human could even be used as a reason for the Nova Corp to look toward earth for recruitment.
The infinity gems teaser was just enough explanation that it won't be completely boring to hear all about them again when they are all gathered in a future movie (which would also require a history lesson of the gems). It was also the most information we've had on the gems so far.
Also, I think I could be the only person who thought the stinger, with Cosmo and Howard, was funny.
My biggest complaints were about a couple moments of very cheesy touchy-feely dialog that could have been removed or altered, but they weren't too bad or too often that it detracted from the film as a whole.
I would have liked to see the Nova Corp actually use the Nova force, but I did like their ships.
Gamora was used poorly in several moments of the film. I came away feeling like she was a badass, but definitely not the "deadliest woman in the whole galaxy". We had a great actress, but the part waned on the old standard of girl in distress. Other than Drax showing up, there was no reason Gamora couldn't stop those prisoners trying to kill her (and maybe even then).