Hardware and software compatibility.
Bought my first mac ever, put in SSD + 16GB RAM ...... then realised I have £1500 of sw/hw I could use on XP, Vista, 7, SL but not ML.
Bummer
I'd gladly pay for a solution to get SL on Ivy Bridge MM.
(there's 100's, if not 1000's who really could do with this solution)