So some interesting stuff which most probably won't be considered spoilers.
Weapons and technology (well this is a site dedicated to such)
The Dark Elves have guns and a "vortex grenade". Which sucks you into a vortex and goodness knows what it does after. The Elves also have a ship armed with a cloaking device. Heimdall managed to sense one of them though.
Among the Asgardians, only Odin has the equivalent of a fire arm. His spear which shoots energy beams. However the Asgardians have anti aircraft guns and mention artillery and also their own equivalent of jet fighters, which engage the Dark Elves ships.
Motivations
The motivation of the bad guys were to bring about the return to darkness, which the Asgardians think is to change normal matter into dark matter. Geez, what 96% of the universe is dark matter and these guys bitch about the remaining 4%. What selfish gits.
Loki's motivation to help Thor is revenge. Given that in adverts it already shows Thor seeking Loki's help, this isn't spoiling it.
The Dark Elves
Its nice to see that the Dark Elves have their own language. The only time they speak "English" is when they interact with others.
Their costume is also nice indicating how "alien" they are. Further more when they fight they wear masks, which I feel captures the feel of how alien they truly are.
Oh, and in the comic Malekith needed a great spell to turn Algrim into Kurse. In the cinematic universe they could turn any elf into a Kurse. They don't seem to be able to do more than one at a time, maybe the method is limited or its very hard to do 2. We see them do it when Odin recounts the old battles between his father Bor and the Dark Elves, where the Kurse then is overwhelmed by several Asgardians.
Christopher Eccleston has starred in some stinkers, even though he was allowed to shine in those. I am mainly thinking of the Dark is Rising and GI Joe. He does a fair job as Malekith in this, although his charisma doesn't shine through, because of how Malekith is, well cold.
Spoilers
Thor and Loki's interaction Spoiler