Ugh, I HATE people who put their feet on seats! It just shows a complete and utter lack of respect!
When I see people putting a newspaper down and then putting their feet on that, it's fine, as they obviously are showing consideration that they don't want to make the seat dirty.
And people taking their shoes off first doesn't bother me, even if their feet are a touch sweaty, it'd soon dry out, and wouldn't mess up the next person's trousers like mud from shoes would...
That said, I've never felt so uncomfortable on a train that I've felt the need to put my feet on the seat...