At a quarter to five on November 2, 1982, little strips of neon spun across millions of television screens to a sonorous four-note tune that eventually everyone in the country would recognise.

A few minutes later, a chirpy northerner wearing a silly tie asked for some consonants in a quiz show that eventually everyone in the country would play.

Then a few hours later some Liverpudians started to be very unpleasant to each other in a soap opera that eventually everyone in the country would love.

It was the first night of Channel 4.

Those strips of neon were, of course, the logo that would became a symbol of controversy and innovation in television; that quiz show was Countdown which became a byword for coziness, Britishness and amateurishness; and that soap was Brookside, which was not only very good but also went on openly toencourage sexual tolerance with the first primetime lesbian kiss.

It's arguable that, along with Big Brother, the logo, Brookside and Countdown are the things for which the channel will be best remembered during this period of anniversary celebration. And while it's right to party, it's also right to ask what has been good and bad about the channel over theplast quarter of a century.

The good first. If the channel has led the way on anything, it is comedy. That first night in 1982, Dawn French, Adrian Edmondson and Jennifer Saunders pretended to be the Famous Five in the first of the marvellous Comic Strip spoofs. As ITV gradually abandoned its commitment to making viewers laugh - except unintentionally - so Channel Four discovered new stars. Harry Enfield was arguably the greatest as Loadsamoney on Friday Night Live in the late 1980s, but there was the unfeasibly brilliant Father Ted, too, not to mention some of the best comedy imported from America, the best of the best being Frasier.

On drama, too, the channel has been a pioneer and a great discoverer of talent. One beacon was Queer as Folk. Not only did it push tolerance into the faces of the intolerant, it was also compelling drama. Russell T Davies wrote a sparkling script on his way to the top of the profession and further success with Doctor Who on the BBC.

Queer as Folk was massively controversial, of course, because of its sexual content - and controversy has rightly been the theme of much of the anniversary celebrations. This desire to provoke sometimes has led to mistakes - Brass Eye's misjudged satirical programme on paedophilia, perhaps; the execrable Alternative Queen's Speeches with Marge Simpson and Ali G, certainly - but it's funny that the channel's biggest hit has not been a controversial one, but a nice, cozy one: Countdown. The presenter, Richard Whiteley, died in 2005, but his co-host Carol Vorderman is still there. On that first night 25 years ago the programme with its stick-on letters and dictionary corner was dismissed as naff, but viewers embraced it. Channel 4 may be a little embarrassed about it, but it shouldn't be.

There are many other good things about the channel. The movies it has supported: Four Weddings and a Funeral, Shallow Grave, The Crying Game and The Last King of Scotland among them. And, of course, Channel 4 News, still the best, most appealing and most real news programme on television. And The Tube deserves a little PS; in a stroke it made Top of the Pops look like a diplodocus.

The bad chiefly means Big Brother, which might have started out in 2000 with the stated intention of being a social experiment but over the years descended into the kind of programme that allowed the alleged racist bullying of Shilpa Shetty.

Of course the channel created many of the formats and ideas that later became accepted by the other channels. Indeed, the defence of Channel 4's failures has always been that it is a channel which pushes the boundaries. But could the opposite now be true? Over those 25 years, perhaps it has become a little too commercial and not enough concerned about providing an alternative voice. Then again, in a world where viewers now regularly have the choice of 30 channels rather than five, the channel has had no choice.

Yesterday, the head of programming, Julian Bellamy, said it was time for one of the channel's periods of renewal. "Channel 4 has always had the uncanny ability to reinvent itself before it becomes too samey," he said. "I believe that in the first quarter of 2008 you'll start seeing the manifestation of our redrawn manifesto. There will be more new things than in 2007."

It would be nice to believe that. A talent for innovation is the thing Channel 4 has most lost over the past few years, and the thing it most needs to rediscover.