Looking at the tech specs for Apple's latest iMacs, all the 21.5" models seem to have 5400-rpm hard drives unless you specifically upgraded yours. (I can't find a 3.5 GHz model actually so I couldn't verify your model and I also couldn't comment to ask.)
Based on the assumption that you do in fact have a model with a 5400-rpm dive, your boot time doesn't seem unreasonable. Of course, the fact that you posted about it begs the question if your boot time was much shorter before Yosemite?
An SSD (or maybe a Fusion drive) would drastically improve your boot time (and probably overall performance as well) compared to a 5400-rpm drive.
Your boot time might also improve if you do a clean install of Yosemite. I guess part of the reason you chose to upgrade is that you, like me, have got too much stuff that would be a hassle to reinstall? Having many applications and files on the HDD I believe could also affect boot time since the OS probably does some indexing and who-knows-what-else to the file system when the system is booting.
TL;DR: If you have a 5400-rpm it's not unreasonable for boot times to be this long. Clean OS install might help but the biggest improvement would be seen with an upgrade to SSD.