We've given them Pop Idol and Strictly Come Dancing, but last night, after a 50-year wait, we took one of America's most time-honoured TV formats and, against all predictions, did it better than they ever have.

The first British leaders' debate aired to a grumble of low expectations. It would be dull, the conventional wisdom forecast, bound and gagged by the 76 rules demanded in advance by the politicians. Even David Cameron warned it would be sluggish. There would be no clashes or direct confrontations, just the dull recitation of party programmes.

That's how it often operates in the US – long stretches of tedium interrupted by the odd spark of conflict. But that's not how it turned out. Cameron, Nick Clegg and Gordon Brown imported a dash of Westminster combat into the proceedings, addressing each other directly, demanding answers, mocking their opponents. The result was a debate that surpassed expectations.

But that was not the greatest surprise of the night, an honour that went to the Liberal Democrat leader. From the start, Clegg asserted himself as the star of the show, anointed as such by a whopping 51% of those surveyed by an instant Sun/YouGov poll.

The first shot had him looking alarmingly young, boyish and eager, but he soon transcended that. More than his rivals, he demonstrated an instant understanding of the format. All his answers were delivered to the camera, since that was where the audience that mattered was to be found. He addressed questioners by their first name, a habit later picked up, though less naturally, by the others. He was studiedly colloquial – "You won't believe this, Jacqueline" – and cast himself as the rebel. "Apparently I'm not allowed to ask you questions," he said to one member of the audience, "so just nod your head."

Of course, his position was easier than the others. As the leader of a smaller party without a sniff of government for 65 years, he carries less baggage. But he exploited that advantage with verve. His opening answer referred to "these two", lumping Brown and Cameron together as the double-headed representative of politics-as-usual. "The more they attack each other, the more they sound the same."

But instead of closing ranks and crushing this impudent upstart, Cameron and Brown fell over themselves to win his support. "I agree with Nick" was the catchphrase of the night, an early public sign of the post-election haggling that may ensue if we end up with a hung parliament. Brown repeatedly tried to cast himself as Clegg's ally, desperate for the two of them to pile on Cameron the way Barack Obama and John Edwards once did against Hillary Clinton. But Clegg was playing hard to get, audibly snorting when Brown claimed he and the Lib Dem saw eye to eye on political reform.

The instant polls put Cameron second, and it's true that he maintained his usual fluency, explaining policy ideas in simple, accessible language. He was able to connect policy with real life, answering a question on the NHS by citing his personal experience with his late son. Plus he was easy on the eye and ear.

The trouble was that all those same skills were displayed by Clegg, who had the added benefit, for most, of novelty. Cameron was out-Cameroned by Clegg.

The Tory leader may also have stored up some trouble for himself. Last night Labour was pouncing on Cameron's bracketing of China alongside Iran as states that pose a nuclear threat. In a US debate, that would be hailed as a major gaffe, showing that the challenger was not ready to be commander-in-chief.

As for Brown, he was on top of the detail, and solid. But he looked tired and rumpled, often glowering. Above all, his answers were too packed with indigestible policy detail. He failed to understood what Clegg intuited perfectly – that TV debates are aimed not at the logical, but at the emotional part of the voter's brain. A long line of losing presidential candidates could have taught him that lesson.