I suppose this was a moment of political and television history - the first time in an election when we have had the leaders of the three main parties debate with each other. It was a bit slow at first but warmed up as they began debating with each other as well as answering audience questions. I suspect, as with the televising of parliament, that over time the rules will relax and conventions will develop.
It wasn't surprising that Nick Clegg did so well in the polling afterwards since people were aleady familiar with Gordon Brown and David Cameron. Cameron probably disappointed his own side. He did not come across as convincing and was not relaxed. He was on the back foot at times because he would not match Labour's commitment to maintain spending on the police, on education, and on key health services. Brown was the clear heavyweight, with the most gravitas of the three.
We mustn't forget this is one of three debates. It might be the most important as it was the first one. However the other two are unlikely to be complete repeats. All candidates will learn from their experience.