Joss Whedon has confirmed that Loki (played by Tom Hiddleston) will not appear in The Avengers 2. He is quoted as saying,
"First of all, imitating what I did before is the surest way to do it not as well. Second of all, Loki’s not there to say those terrible things. Although I do think we should bring the word back, not as an insult, it’s just a nice word.”
So there we have it. I can respect that Whedon isn't relying on the tricks and humor that has worked in previous movies and wants to give us something new. I also think with the villains they are introducing are badder and a bit beyond the scale of Loki, so he is the least of their worries at this point.