Twenty years from now, people will still spend 3-5 hours a day passively zapping the tube. It's as inevitable as day and night. But they'll have as many channels as there are websites today. There will be a few hundred really big channels, hundreds of thousands of big ones, millions of local and niche channels, and billions of low fi personal channels showing pets on skateboards.
The simplicity of TV - switch on, zap till happy, watch till exhausted - will always beat a more complex model and will always define the "TV" experience. But people also value choice and control. So, obviously: pause, stop, rewind, resume, etc. Huge libraries of commercial content. Free weekly episodes, padded with advertising, and more episodes for a small price. Multilingual content as a matter of course, even, eventually in the UK. Real time content - sports, events - will draw huge crowds, like today. But imagine a hundred cameras - each a channel - in a stadium, zooming and panning automatically, mixed with semi-pro commentators with their own channels, and you get the televised sporting experience of tomorrow.
Every webcam will come online. Want to check out who's breaking into your neighbour's house? Switch on the TV and zap to your street's webcam. Something like Google Earth is going to feature strongly in your future TV's user interface. Also, what's up with those buttons? The future TV remote control is gesture-oriented. Flick to the left and down, pause and open new window.
The large screens - no-one has anything less than a 3x2 foot screen on their wall in 20 years - makes new kinds of UIs plausible. Lots of channels on one screen, each in a window (yes, we'll still have windows in 2027), sized to show importance, automatically drifting into motes of irrelevance if they're not being used, main window enlarged in middle of screen or fullscreened. This would be a great UI for other stuff too.
TV content won't change much - there are only about sixty storylines in all fiction, say some. Sex, violence, horror, and drama will keep people glued to their tubes. But also, peeping into the lives of other people… the humble webcam is the predecessor of the future of TV, in large part. Social standards of permissiveness are not infinitely elastic, and basically most of us are prudes, but it'll be possible to watch anything, at any time of day, if you know the right passwords. The criminals will play a big part in providing illegal content, where "illegal" can mean anything from accurate news (in repressive states) to pirated episodes at a discount price.
Ultimately the market always wins. The MPAA and RIAA will one day embrace digitality and understand that the digital choice opens many more wallets than digital scarcity. But I suspect it won't be the traditional content providers - today's TV stations, newspapers, movie studios - who rule the TV in two decades.