I imagine the 300D or Rebel as its known in the USA may show its limitations if you're taking a quick succession of touchdown shots or perhaps some air display type shots. It's in these kind of situations you wish to take multiple shots in quick succession to get the best one, and the 10D is much better in these situations than the 300D. Also, the 10D can cope with faster focusing too - probably not an issue for most of us, but if you intend to use your camera for non-av work where the subject is moving quickly at fairly close quarters, you ought to take note.
For many people here, I'm sure the Rebel/300D/Kiss (dependant on where you live!) will be fine and it is for sure a very good camera, and I hope that it may be for you too. I myself will be going for the 10D as they'll be times when I might need those extras and I take the view that I ought to buy the best camera I can afford. It also has a metal casing rather than a plastic one.
A final piece of advice - even a 300D/Rebel is a major investment and you should understand some of the terminology in order to understand what the camera can do and cannot do for you. I'll never get to grips with all that computerspeak but I always advise people to read the web, as stated elsewhere in this thread, and buy a magazine or two that explains these things in context.