Again,
#1 Feature: Multiple monitor support. If you only use a single monitor then yes... no need to upgrade because of this. (New AirPlay has some merits though).
#2 Energy savings, App Nap & Compressed memory: This is more in line for those in the market for a new Haswell-based MBP over the next few months (like me). Can't see anyone upgrading an iMac because of this. And... after using Mavrix on an early Sandy Bridge machine for a month now I can honestly say there is no noticeable battery savings for me. At least none that i see. (I think it's slightly worse but not sure).
That is pretty much it for me.
Finder tags/tabs... never use 'em.
iBooks... whatever.
iCal (Calendar)... I use BusyCal
iCloud Keychain.... might prove interesting. Haven't tried it yet.
Notifications... nothing earth-shattering.
Maps... meh. I never get lost.
So no, you mac won't burst into flames if you stay on ML.