manual focusing L lenses?
It's very difficult to manually focus on modern DSLRs, as the viewfinders are designed to be as bright as possible, which sacrifices manual focus accuracy. Check out some of the replacement focusing screens if your camera supports it (I know my 5d supports it). Some of the replacement screens, like the EG-S for the 5dII make manual focusing MUCH easier.
Other than that, if you plan on focusing manually, one minor annoyance is focus-by-wire. I know the 85 f/1.2L II and 40 f/2.8 pancake have focus-by-wire, which means when you twist the focus ring, a tiny motor inside the lens actually moves the focusing mechanism. It's a little tricky to use if you're used to manual focusing on other lenses. Also, the camera body has to be powered on when focusing.
Just some things to think about.