With regards to why the game wasn't just based on the SC/FA engine, it is because the engine or underlying architecture that governed how resources were generated was extremely complex and dogged FA down in terms of performance.
I had read a C.Taylor interview about this, where he explained why he had to move away from the SC/FA era. Almost everything in FA had a constant resource tick, even vehicle/object debris (until properly destroyed) for example. FA, lets face it was a dog in terms of performance, let alone AI but the core gameplay was rather sound.
I feel somewhat dissappointed with the short amount of SC2 I have experienced, but I'll play the remainder of the campaign missions later tonight.
I've always been an RTS tart, but firmly in the Age of Empires/C&C/Total War games. Prior to a mate mentioning SupCom in 2009, I had never even heard of the games. Then I was introduced to FA mid-2009 and the way I looked at my other RTS games completely changed.
I still can't get over how unbelievably camp the ingame 'videos' are of the various characters. Don't get me started on the quality of the ingame units we've seen so far...