When QI creator John Lloyd was asked to lay out his programme's manifesto in The Idler, he began with this directive: "Everything is interesting. Allow yourselves the luxury of looking clearly and patiently at anything – a turnip, the history of Chelmsford, a letter from an insurance company – and new layers of detail come into focus." With the show now in its tenth series, that luxury is still enjoyed by a very healthy audience: four million viewers watched the first episode of this series, if you combine the figures for QI and its extended repeat QI XL. So what keeps us tuning in?

It was 2003 when everyone's favourite headmaster, Stephen Fry, introduced the first edition of the show, entitled Adam. Now we've reached the letter J. Fry has been joined over the years by regular team captain Alan Davies and a revolving panel of funny, intellectually adventurous guests such as Dara O'Briain, Bill Bailey, Jo Brand and David Mitchell. They talk around subjects rather than trying to score a direct hit with the right answer.

And, while other long-running series have started to look a bit knackered, QI still manages to surprise, as a guest pops up with an unpredictably deep knowledge of a pet subject. I loved Ben Miller waxing lyrical about quantum physics – he wouldn't be allowed to do that on The One Show.

After all this time, QI also still has the power to tick, with a flourish, at least two of Lord Reith's dusty old boxes: entertaining and informing by the barrow-load. Last week the show even seemed to address the constant criticism that women are under-represented on panel shows by including a record three female guests in Sue Perkins, Jo Brand and the exquisite Liza Tarbuck. Incredibly, the sky didn't fall in. And it was really funny. The producers plan to keep the supply of funny women coming in future episodes too.

Rather than working like a straightforward panel show, QI gives participants the chance to freely associate, to chatter and to toss their idea-tomatoes around in the collective salad bowl. Points are awarded for intriguing segues rather than straight answers. And that's the key to its success. You feel like you're at the pub with the funny, clever people, ear-wigging on their slightly tipsy meanderings, rather than standing against a wall while they fire their joke cannons at you. It draws you in, all that familiarity and casual pontification.

The badinage doesn't ever degenerate into combative cock-fighting as it sometimes seems to on shows such as Mock the Week. The panelists tease and rib – with particular focus on Alan Davies who always gets the most demeaning sound for his quick-fire buzzer – but it's camaraderie and intellectual kinship that parps warmly into our living rooms from QI, not the chill, stale gas of more aggressive offerings.

TV peers such as Never Mind the Buzzcocks might have their moments, but there aren't many panel shows on TV where comedians competing for screen time seem so happy to let each other speak. In fact the only recent show to equal QI's all-round pleasantness was Channel 4's Paralympics companion, The Last Leg with Adam Hills, which also hit just the right balance of cordiality and inter-comedian goading.

Meanwhile QI is one of the last truly popular programmes on mainstream television where comedians are allowed to be clever or even vaguely cerebral without being ghettoised into some late-night shame-slot on BBC4.

True, not everyone wants to listen to thoughtful, articulate people demonstrating their skills. Some might see them as smug showoffs who should keep their big brains and large vocabularies to themselves. But thank goodness for series J of QI. The BBC will be failing in its duty if it doesn't let them get all the way to Z.