I just finished watching the debate and I'm going to post some of my dull thoughts on each party leader.
Ed Miliband: He was the only one on stage who spoke like the leader of a country. He's the most authoritative and self assured. However, he damaged himself and his party every single time he mentioned Cameron or the Tories, and he did that very, very often. He started off strong, but weakened in actual debate. He should have promoted Labour as the best party, but he ended up promoting them as the party that isn't the Tories. I felt, however, that he still gave of an air of great intelligence. He's a bit of a geek, as he's not the best at representing himself in public, but he's a smart guy. He came off as a somewhat conservative because he was placed on a stage next to three more left wing parties and one raving lunatic. His policy seemed measured but not decisive. On the whole, it was an okay performance for him, and the only reason he didn't do poorly was that Cameron refused to show.
Leanne Wood: She wasn't very memorable. She's not personable enough, and her delivery is too practised, almost forced. It sounded like she was reading from notes in front of her rather than speaking from her head. However, she represented her party fairly well, and her performance was consistent throughout. Her policy seemed very similar to that of the SNP and the Greens, to the point where there seemed like there was little differentiating her from those two. What was lacking was emotion, and I think with a bit more passion she would have delivered a more detailed representation of their manifesto. Overall, however, she gave a fairly forgettable performance.
Natalie Bennett: I felt that she did a very good job in this debate. I'm biased, admittedly, in that my views line up most closely with the Greens. I felt her policy was well represented and that she was one of the best at actually debating. She said the right things and said them well. However, I did feel that she seemed a little inexperienced on stage. In stark contrast to Miliband's confidence, she seemed a little unpolished in her manner. By the end she was doing better, but overall she didn't exude the air of experience one would want in a leader. That's not to say that she did poorly, but she doesn't seem as experienced as the others. She was still my favourite, but I'm not sure I could credit her with a win.
Nicola Sturgeon: Strong performance here. Like Natalie Bennett, her policy was clear and well represented, and she did an excellent job of debating on stage. She also seemed calm and confident. I can't think of much that I can fault her with, really. The one thing she did too much was mention Cameron and the Tories, though she wasn't as guilty of this transgression as Miliband, and she did it in a way that still made her look strong. I'd give her the debate win by a small margin.
Nigel Farage:

What an idiot. He insulted the panelists and the audience, and even the BBC. He really wasn't on the same level as the others. He didn't behave like a statesman or a politician or a leader, he behaved like a child. To call him the loser gives him more credit than he deserves.
David Cameron: He was the whipping boy for the entire debate. He looked weak and cowardly for not appearing, and he was the real loser here. Not appearing on the debate was an appalling political blunder, and I wouldn't be surprised if it costs him the election.
Nick Clegg: Who? The debate went on as if the Lib Dems didn't exist, which highlights their lack of identity separate form the Tories. They have a lot of work to do in the next five years to build up an image and an awareness. As it is, they're not really a player here.