I take it from other reports that I'm not the only one that crashed during setup on FileVault. Incidentally, my system was on 10.10.2. In the process of updating now.
Mine (1.2/512) came with 10.10.2 installed. FileVault went off without a hitch whatsoever. No delay, no beachball... As soon as I booted into OS X, I updated to 10.10.3. FileVault is on, and all seems well.
Same thing happened here a well. I wonder how this escaped their QA process if it's happening on so many (all?) of them.
I just restarted, created a second admin account and went back and deleted the second one. FileValut enabled no problem, and there was nothing weird after I upgraded to 10.10.3.