On computers at, below, or even a bit above the minimum spec for the main installer CD (live CD) for feisty, getting as far as the live CD desktop is horrendously slow. The solution is to use the alternate install CD, but no-one knows this (you get an option to download the alternate CD but don't get told *why* you might want to). Can the main CD either check specs on bootup or time how long it's taking to boot and then, if that time is over a certain threshold value, put up a message saying "this machine is probably too slow to use this live CD installer, although it will be fine to run Ubuntu: we recommend that you download and install from the alternate CD, available from http://wherever/".
For extra bonus points, on the download page, attempt to time the computer that's about to do the download with a bit of JavaScript and if it looks like it's going to be too slow, say "If you're going to be using this CD to install Ubuntu *on the computer that you are on now*, then we'd recommend using the alternate CD because this machine might be slow using the ordinary CD." This obviously isn't a perfect test but it should help at least some people not download the wrong CD first time!
I think the main constraint here is memory, which should be easy enough to check.