Wasn't the much-cited fact at one point that the reading age required for The Sun was one year lower than that required for The Beano or Dandy (children's comics, for those who are unfamiliar with them)?
Mastermind, as mentioned by PP, is an interesting case in point. It calls itself 'TV's toughest quiz', which is clearly just silliness - as Only Connect and University Challenge, to name just two, are manifestly much tougher.
The two Mastermind rounds are starkly set at two very different levels: the specialist subject round is indeed often tough for the typical person (presumably, that's the whole point); but the general knowledge round often features laughably simple Tipping Point-style questions.
I agree that the pressure and the TV cameras are obviously a big factor; although, interestingly, the performance level of the editions featuring celebrities - who are well used to TV cameras and lighting - is usually significantly lower than the editions featuring ordinary people for whom it's probably their first/only ever time on the telly.
I find The 1% Club scary, assuming that the percentage levels given are accurate. Not even so much the very early questions, as you're obviously always going to get some people with learning difficulties, underprivileged, academically challenged, poor eyesight, not reading the question properly etc. - although you clearly also have to counter that with the guess factor, where the cliché monkey jabbing at one of two buttons would unknowingly accidentally get the right one half the time.
It's the 70, 80, 85, 90% questions that frequently surrprise me far more, as they often seem so straightforward and uncomplicated, regardless of average individual ability. My 12yo DS loves to play along, and frequently (not always) gets the later questions quickly. Yes, he is bright... but he's also only 12, and it's supposedly (unless I've misunderstood?) based around and aimed at adults.
The culture of many people not actually reading at all these days must play a large part in it; distraction too, as there are so many media and other things vying for our attention.
I sometimes enjoy Enid Blyton books as a guilty pleasure - Famous Five and Five Find-Outers rather than Noddy (as if that makes it much loftier!!).
Whilst there was obviously a level of assumed privilege and access to books and education in her writing, they reflected a time when people's 'world' was so much smaller and boundaried and far more local. It was much easier to be able to focus on a particular task, book, hobby or specialism, if you put your mind to it.
Nowadays, although the knowledge and learning opportunities made accessible by the Internet are myriad, they are so myriad that you can easily feel like a very tiny little fish in an infinitely enormous pond. How long would it take you just to read the archive of Chat threads on MN, let alone the rest of this one site, or all discussion fora, or to consume the content of the entire WWW? Literally thousands of lifetimes.
Whilst it may seem surprising that so many people happily boast about never reading a book or being rubbish at something quite basic, I think it's partly defence mechanism, but partly a result of living in an age where the entire world is at your fingertips and 'overwhelming' is about as gross an understatement as there could be.
We used to have a lovely but daft old dog in the family, who would get bewildered by the different options that he wanted to grasp - but obviously couldn't - all at the same time. Several people there to be made a fuss of by, lots of nice-smelling food and treats, balls to chase, toys to play with, interesting and curious sights and sounds to investigate etc. So much rabbit-in-the-headlights sensory overload and so many options that his frequent response was to choose none of them and just to flap around and wee on the carpet instead!
Although humans are undoubtedly far more sophisticated and multi-faceted than dogs will ever be, I can very clearly see the parallels in our lives, experiences, challenges and responses (granted not actually just wetting ourselves as adults!) - in the face of such a massive, boundless and bewildering world. With literally millions of years' worth of reading available to 'complete', I can very well see how many people just don't have any notion where to start, and thus don't ever start at all.
Sorry, that ended up very long and rambling!