Here is what I thought:
His big plus is that he sounded in control, knowing his staff and having clear answers. He sounded convincing. He wanted to project competence, and he somehow managed to do it. Though this is no surprise - all questions were carefully screened, answers prepared and the chosen questions were non-confrontational - but it is still the 'being in control' thing matters. Brown's performance did not give the impression that the country is out of control and on a downward spiral, which it is, and on this count, he did better than expected.
I think Brown's big mistake was that he sounded too political. At this time of scepticism, competence is not enough. He failed to project labour as a team - for example, on the question of immigration, he could have projected Alan Johnson as a Home Secretary who tightened up the system unprecedentedly - and talked too many times about 'I' deciding on everything. And, in his eagerness to steal some of Lib Dem votes, talked about Nick Clegg agreeing with him on too many things. Worse, in some cases, he stole Lib Dem ideas and said Lib Dems agree with him on those - not something which this country averse with politicians is going to take in well.
Brown's body language was composed, he drank least amount of water and despite his physical difficulties, managed to project ability and composure. He wanted to and sounded Presidential; a calculation his advisers must have made about the country wanting someone safe and competent. He was a bit uncomfortable when talking about Afghanistan, he made a few slips there, but was overall on the point.
David Cameron, on the other hand, sounded flighty and stiff. He started drinking water all too soon. He was ill at ease on most questions, and almost always tried to change the subject. His big problem is that he has no team, and the other two leaders, perhaps by agreement, did not want to expose that weakness. But he could not make up his mind what he wanted to do, and fared the worst among the three.
The other big problem with Cameron is that he fails to understand the country is not a company, or a family. This is a different time than Margaret Thatcher's, when people were tired of government control over everything. Today, we feel the reverse - we believe government is too loose. So, talking about government waste and bureaucracy are going to evoke a much weaker sentiment. The deficit is bad, indeed; but if Cameron fails to explain why he thinks deficit is bad, and how this may bankrupt Britain, he is unlikely to get the votes that he is wishing for. Instead, he will sound too cosy to the bankers, and that is not a good thing at this time.
Lastly, Cameron appeared to be less of a people's man than a practitioner of spin. If he needed to talk to the person who asked the question, he should have noted where that person is sitting; instead of losing the person in the crowd all the time. Nick Clegg did much better noting the names, and acknowledging the obvious difficulty of spotting the person while being in the limelight. Gordon Brown did not even try to be that personal.
I would think he did the best in debate terms, but he failed to disperse his 'lightweight' problem. He compounded that mistake by going straight after the Trident programme. That bit was antithetical to his otherwise sterling performance, where he failed to explain that he is only talking about the scale of the programme and not Britain's nuclear deterrent itself. David Cameron and Gordon Brown stole a few important points ahead of him on that issue. While Nick Clegg could not have done anything else, given his party's anti-nuke stance, the issue, which he only raised, showed that he is not ready to rule.
His body language was perhaps the best among the three leaders, and his communication was clearer. His job was easy too - his party was never in power and gets none of the blame. He had some good ideas, and he could have done better that some of those were being stolen by Labour or the Conservatives. His economic calculations have more meat, and his team is perhaps more capable - he could have done well talking about Vince Cable and others, and show that his crew is different from the other two Presidential parties.
I know this is only one of the three debates, but there are certain questions and answers which I think Brown missed the point completely.
David Cameron said that migration never exceeded 76,000 a year whereas during Labour's term, it was never lower than 140,000. Brown could have said - It is globalization, stupid!
Clegg and Cameron had talked about Ministry of Defence having a lot of people in communications. Isn't that because the nature of Britain's engagement, and the nature of war itself, has changed?
In short, this shows Brown is no Berbatov, too many misses.