i've got the feeling the beta slashdot was designed by someone who makes websites for the elderly
Not quite. It was designed by someone who (sadly) knows what they're doing, and more importantly knows what advertisers currently want.
A lot of sites have shifted over to this kind of design, so Slashdot is far from alone. Advertisers like it for various reasons - mostly it comes down to ads being more eye-catching - and of course advertisers control the purse strings. But at the same time the public has also had it ingrained into their heads that such a design is the definition of "modern", and therefore any site not using such a design is by definition not modern, and therefore not worth their time ("the site's owners can't be bothered to keep up with the times?"). Which just leads to a feedback loop of advertisers liking it even more, since they want to advertise on sites that the public perceives as hip/modern so that their products are seen in a similar manner.
But the truly terrifying thing is that the industry isn't done yet. I can show you what the future will look like and it isn't pretty: http://www.theverge.com/2014/1/21/5307992/inside-the-mind-of-a-fanboy. By all accounts the advertisers seem to love it; the text is big and readable (i.e. not threatening), there's animation and lots of flashy effects, etc. It basically reads and behaves like an interactive glossy magazine (People, not Popular Science).
Not that I agree with any of this. Beta needs to be buried 6ft under in a grave lined with lead. But there is a method to their madness; despite what a lot of people here think, Dice hasn't gone off of their rockers. The rest of the world is just slowly moving on from text to images and iconography, as sad as that may be.