For me, when hitting submit a link/text post, you have to fully type in the sub. How about typing in the first letter or two will bring a list with subs that start with those letters?
Ex: Type in P and O, and Politics, Porsche, Political_Revolution, etc. come up in a list
I think there must be a better way but this is definitely a good start. My main frustration with this sort of system is that you can't discover subs that you don't look for.
For example, you would have to guess that a sub about cats would be called "cats," or possibly "kittens," or "kitties," or "pussycats." So you have to try several words to see if a sub exists. And you end up missing the sub called "felines" and "hypnocats."
A better system might have subject tags or categories of related subs. This would help people discover subs without having to guess that a sub exists and also guess its name.
Additionally, sub names don't always match the topic or give clues to the topic.