Starting with Straczynski's Thor and all the way through Dark Reign we see that Loki is manipulating the events behind the scenes, and all this leads to the attack on Asgard. Loki is using Norman Osborn to instigate the siege, but then when it's actually happening he suddenly changes his mind and actually dies defending Asgard. The explanation Bendis gives for this is that Loki didn't realize how powerful Sentry was, and he didn't want Asgard to be completely destroyed. But there's still a couple of big questions left unanswered: 1) Loki has been observing the Dark Avengers for a quite some time, plus he's a freakin' god, so shouldn't he have known the extent of Sentry's power? 2) If Loki didn't plan the outcome the Siege had, then what was his plan? Why did he manipulate Osborn into attacking Asgard? This question was never properly answered, and considering Siege was the culmination of a huge crossover that lasted over a year, you'd think explaining the motives behind it would've been something Bendis & co. wanted to do? Also, in the Siege: Loki one-shot we see Loki manipulating Hela so that she promises Loki won't go to Hell when he dies. You'd think this was an important plot point (maybe Loki planned his heroic sacrifice in advance?), but as far as I know nothing comes out of it.
He did talk over with Osborne that Asgard "didn't belong" in Midgard. Perhaps if, during the seige, the current ruler were killed, then Loki would take power and take Asgard elsewhere (my theory, anyway, but you're right, it wasn't explained). Also...remember that Sentry turned into a freakish monstrosity that a lot of heroes had trouble taking on. Probably not something that happened while he was "saving the day" in Norman's faux-super team.