I don't have a lot of time, so I'll be keeping this review very brief. Basically, this episode kicked some ass. I think it was extremely powerful and important to the overall story. I will, however, start with a few weaknesses, because of course nothing is perfect...
So, Bonnie. I know I've mentioned this a few times before, but her character has made very little sense this past season. I'm not sure what her game is. It was strange to see her making deals with Katherine and hiding things from her friends. She wants to drop the veil, apparently, and that seems to be what Silas wants as well, but yet Bonnie is clearly hiding something from Silas.
And speaking of Silas, he's still a rather weak villain. I don't find him intimidating at the end, when he emerges from the shadows in Caroline's house. However, I was properly scared of him for the first time when he was pretending to be Klaus - then Matt - then Klaus again, in order to scare Caroline. So... he's a mixed bag for me. I'm reserving judgment.
But, the rest of the episode was great. Damon and Stefan are truly ruthless as they try to save Elena, resorting to extreme measures such as burning her, and even getting Katherine to try and torture some sense into her. The sunlight scene was a high point in the episode - the emotions on Stefan's face showed how conflicted he was about hurting her, and Damon's callousness was clearly all an act. Elena is truly insane without her humanity- she actually lets herself catch on fire to prove that since she's the girl they love, they'd never let anything really bad happen to her.