It's certainly far from impossible, though.
The first thing you have to do when switching from tanking or healing to DPS or vice versa is abandon how you approached the job. You're not doing that job anymore. When I first went DPS in Firelands, it took me two weeks to get myself to stop trying to intercept mobs running for the healers or other DPSers and getting myself killed. That was because no one was healing me -- not because they didn't care, but because they had no idea I was about to get aggro on Firelands trash. Why would they? I wasn't a tank. It's not that they didn't appreciate it; it's that they had no way of anticipating I was going to do it.