Loki doesn't teleport.

Many fanfictions portray Loki as popping up and off at a moment's notice, but canon shows us something different. Thor implies that Loki had some way of getting around to other places if he really had to, but the fact that he immediately switched over to using the Bifrost after becoming king suggests that these methods aren't exactly convenient. When Thor recruited Loki to help him get to Svartalfheim in The Dark World, Loki didn't use teleportation, but instead took them to a portal that lead to Svartalfheim. Loki, from what we learn in The Dark World, is aware of the locations of places like these and uses them to get around as he needs to.

If Loki were able to teleport as easily as is often portrayed, surely Thor would have said something about it when they had him in SHIELD custody - after all, he saw Loki appear in front of him when he was held by SHIELD in Thor, and he didn't seem too surprised, which indicates that he was well aware of Loki's methods of getting around even then. And if Loki was able to teleport on a dime, he would have never needed to get an eyeball so Hawkeye could break in and take the iridium - he could have simply teleported beyond the door and grabbed it himself.

Loki can be quite polite and mellow when the situation calls for it.

Loki is a complicated fellow. He can kill without a second thought. He can break someone down just by verbally attacking their weak points and enjoy every minute of it. When pushed too far himself, he nearly breaks down in tears. But he can also be genuinely polite and soft-spoken, and can even relax a little and enjoy himself. If his lines in the underground lab are any indication, he's even in the habit of saying "please" to his subordinates. (However, if the cut scene in Thor is any indication, he isn't above playing petty pranks on them now and then, either.)

Asgard sew Loki's lips shut? Maybe not...

Many people have the Asgardians punish Loki by sewing his lips shut - and exactly how he's supposed to eat or drink is never addressed. But perhaps more importantly, is this something MCU!Asgard would actually do? In Norse mythology, Loki's lips were sewn shut by dwarves whom he made a bet with, not his own people. This also happened in the comics at one point, but then again, there are significant differences in Loki's characterization between the different media. Add to that the fact that Odin believed Loki dead - it would take a complete monster to easily dish out such a cruel and unusual punishment after such circumstances.

UPDATE: The Dark World reveals that Odin is very callous about Loki, but even then he didn't actually sew his lips shut. He initially wanted him executed, but at Frigga's behest sentenced him to life in prison instead. Cold, yes, but nothing particularly bizarre or unusual.

Loki is not "thousands" of years old.

The beginning of Thor explicitly takes place in the year 965. As of the time this was originally written and when The Avengers presumably took place (2012), that would be approximately 1047 years ago. When Odin found Loki, he appeared to be about three months old. Based on the fact that in the films the Norse stories were inspired by the Asgardians, and that Loki and Thor were very much adults in these stories (the illustration Dr. Selvig shows depicts Loki as an adult), it would seem that neither Aesir nor jotnar take extremely long to mature. Furthermore, we learn in The Dark World that Asgardians apparently have a lifespan of 5000~ years, which would definitely put a cap on Loki's age potential.

It's not established that Loki's magic was seen as "girly" in Asgardian society.

This is another one of these things that comes from assuming that Asgardians = Medieval Norse people in space, which is a pretty odd thing to assume given how many differences there are between the Asgardians and actual Norse people. A cut scene in Thor shows the Thunderer deriding Loki's magic as "tricks," but he never actually says anything that would imply that Loki is feminine for them. And then, keep in mind that this is pre-character development Thor we're talking about - he's kind of a general jackass, anyway. And while a cut scene in The Dark World reveals that Frigga taught magic to Loki, we also have to remember that

If you want to headcanon that Loki's skills were less as lesser because they were "feminine," have ye at it. Include it in your fics or roleplays. Just keep in mind that so far, it's not actual canon at this point (late 2014).

"Seidr" or "seidh" is not another word for magical force.

Only tangentially related to Loki, I know, but it pops up pretty frequently. Seidr isn't a magical force you can just take away from someone - it's a specific magical practice that involves altered states of consciousness. Basically, "Odin took away my seidr" makes about as much sense as "Odin took away my ice skating." Go here to learn more about seidr.