Commenting on your numbers 1-3:
1) I did notice that they did try to make the space elves all serious and badass and the Asgardians were goofy. It reminded me of Ghostbusters with the demon lady and the demon dogs which were offset by the goofiness of the Ghostbusters. Of course this movie wasn't actually a comedy so I agree it seemed off a bit.
2) There is no backstory of Sif vs Jane Foster. Sif clearly wants Thor, but he wants Jane and it probably pisses Sif off. What else are they going to do, show Sif's journal that details how she had a crush on Thor since junior high? In the comics they have a history of course but it's really not necessary to know that since Thor wants Jane.
3) Loki is one of the most powerful sorcerers in the Marvel universe:
http://villains.wikia.com/wiki/Loki_(Marvel) The movies really dumb him down, but ultimately in the comics he is kept in check by defending Asgard from outside powers that would threaten the prophecy of Ragnarok, or the cyclic events that would destroy Asgard. He has helped Thor and Odin many times defend Asgard from outside forces and the movie seems to follow that. While he does break bad even more in the comics and maybe they're going with that, he never killed Odin. Maybe the movie could have revealed that he does have limits to his plots but ultimately it is true, he is powerful enough to destroy Asgard and has a part in Ragnarok. That is where it kind of got cheesy, Marvel bit Norse mythology and injected their own flavor into it. Eventually with such cosmic and godly cheesiness, there has to something even MORE powerful than Gods right? And more powerful than that? And so on and so forth. For example:
http://marvel.com/universe/Those_Who_Sit_Above_In_Shadow
The Infinity Gauntlet is more cheesewhiz and I don't think Marvel should have gone in that direction, it's going to be too much for audiences to handle. They should have just kept it like the Spiderman series and slowly introduce new earthly villains and use all of the good storylines from Avengers and not involve too much of the Asgard cosmic crap, just like they've avoided the Silver Surfer (no FF movie doesn't count), Captain Mar-vell, Sentry, Quasar, Adam Warlock, and even Black Bolt i.e. overpowered superheroes that can level planets for fun.