Question about 360 games in HD

You know if you play Forza 2 or any other games in HD, can you see things you can't see in Standard - def? Because I've never ever played any games in High-def.

Someone told me on Gears that you can see like all the dust in HD and it looks really nice.

So are there things you can't see in SD that you can see on an HDTV? Like in Forza 2?

Thanks.