4.1 is a major release from VMware's POV, since they state it will be the last major release that supports ESX. The next version (I'm guessing 4.2) will be ESXi only. It's also a major release in the sense that it is classified as an upgrade, not a patch if you're using VUM.
Minor releases are the updates (4.0 update 1 and update 2) and patches.
vCenter (and SRM) going 64-bit only is nice. vMotion improvements are cool, the storage IO looks sweet, like imagoon mentioned. Scripted ESXi installs is very nice, as is the ability to easily enable SSH on ESXi.
One new feature that I don't quite understand is Microsoft clusters are now supported in HA clusters.