I have a Mac Mini hooked up to my 50" TV, which works great for lots of stuff like movies, but pretty poorly for things with text (menus, web browser). Is there a way to make the default size of pretty much everything bigger without decreasing the resolution and making videos look crappy?
Unfortunately not yet. This is called resolution independence, it was officially announced for Leopard several months before shipping, but was not in the final product. Snow Leopard does not provide this feature.
Maybe in 10.7 ...
In a web browser, you can use CMD-+ and CMD-- for changing size of text.