Hmmm, wrong about yell.com only on the home page, just saw it (my first time) today.
olderairhead
Having JavaScript disabled means your browser will ignore any 'instructions' contained within a web page. Whether this is a Good Thing or not depends on what the page is trying to do. It will stop all pop-ups and most of those 'rotating' ads/banners, and you won't be able to click on any which do appear (as if you wanted to).
As far as the Pprune site is concerned, no JavaScript also means you won't be counted in the 'current visitors'. Everything else here seems to work fine without it.
But there are many other sites which rely very heavily on the JavaScript to work properly. Without it links may be inactive, selecting an entry within a dropdown list will do nothing, and at worst you may just see a blank page.
In IE there is a prompt option - whenever it encounters JavaScript it will ask you whether to run it or not. This is the setting I use at home since I am usually familiar with the sites I visit and know if the script is really necessary or just for 'cosmetics'. Worst that happens if I say no and get it wrong, is I have to reload (refresh) and say OK.
Added benefit of this setting if you're using Outlook is it applies to incoming mail as well. There is never a good reason to let JavaScript run in mail or news messages.
Sorry to ramble. JavaScript is extremely useful in the hands of a good designer or developer but can really screw thing up when used poorly.