"average joe user" who uses IE does not for the most part know what he is missing because we web developers provide him dumbed-down fallback options as an alternative to a screen reading "please equip yourself with a real browser before returning". Only a side-by-side comparison of an average website will reveal what is missing in IE, a comparison which your average IE user will not do (because if he was capable and interested in downloading a different browser he probably wouldn't be using IE anymore).
Only performance intensive applications and those that use cutting-edge features will block IE users completely. Those are generally not commercial products because business and marketing departments rarely feel that a specific fancy feature is worth cutting out a large (but shrinking) portion of the target audience.
Providing this dumbed-down fallback option annoys we web developers to no end. Not only because it gives you a comparatively crap experience, and not only because it limits our ability to express ourselves and make full use of our skills, but because doing so requires extra time that we have to waste and our clients and end users have to pay extra for. In other words IE users unknowingly exact a tax on the entire internet population in order to continue supporting their technophobia. Much like Windows users put an invisible tax on the internet at large by way of their hordes of viruses and bot nets slowing down the system, driving up bandwidth costs and driving down service quality, but I digress.
So you see we have a bit of a catch-22 here: we cannot give you all the things a newer browser could do for you unless nearly everyone uses one, and some people will not use one until they can see all the cool things they're missing out on. IE9 goes a long way toward fixing this problem for us, but it is not available for older computers running Windows XP and even 9 is way behind the curve. Whereas current versions firefox, chrome and safari are available on WinXP and
are inherently faster, giving their users a better experience than legacy IE even on legacy hardware.