Personally, I thought:
The start was massively confusing what with all the names being dropped, planets being shown, all in a very short space of time!
Drax seemed like a complete pansy and never actually looked threatening... why is he there when Groot is seemingly more powerful and the more interesting character...
The fighting seemed quite "weak"... can't really convey why though.
Drax shooting the rocket launcher near the end just seemed TOO slapstick.
The whole "hold hands" nonsense near the end was far too cheesy.
I am surprised at how TMNT has done.
I had similar thoughts about Drax 'The Destroyer' (Of what? Cake?) as well. I really enjoyed the film, but Gamora, Drax and Nebula were just... wrong from a combat standpoint. It's my only big gripe with the movie - James Gunn could do with rewatching The Avengers and taking some notes. The fight scenes in that movie do a great job of showcasing the power level of the various heroes.
You're surprised that an iconic cartoon/toy range has done so well against a pretty niche Marvel property?
Thanos can't be killed. He is the avatar of Death and augmented by her. He can only perhaps be stopped. He cannot be stopped however if he gets all of the gems/stones which is presumably the point of the movies.
http://i.imgur.com/euxaG34.jpg
Quicksilver and the Scarlet Witch are not owned by marvell/Disney but by Fox and there has not been a barrier there. Saying that, not involving the Shiar, Galactus and the surfer would be a mistake (by Fox regarding advertising for a future film) and for everybody else regarding consistency and continuity of plot.One thing I've realised is that we won't see any of the Shiar - despite being on of the big galactic empires - and any of the stuff the Fantastic Four deal with (like Galactus, Silver Surfer, etc) in future episodes, as Fox own all the rights to that.
Btw Thanos always leaves himself open to beating beaten because deep down he knows he's unworthy of ruling the universe, according to Adam Warlock anyway... and in modern times he's become something of an anti-hero and a fan-favourite, but I don't think they'll go with that either - they turned Ronan, who is something of a morally grey character who teamed up with the GotG in the comics into a jobber villain - he's just 'the big bad'
latest story arc shows Thanos in an equal "relationship" with Death having just been made whole by a power higher than that of the ....Tribunal. It may be that he doesn't feel himself unworthy any more as a consequence. The story arc also reveals a more powerful and aggressive Adam warlock (Source: Infinity Revelation).