I hear you. Motor City Online (when it came out, they later patched it) had a brilliant upgrading/tuning system where you had to buy parts without knowing exactly what effect they'd have. You knew more or less what you'd get (bigger wheels improved acceleration, while smaller ones gave you more speed) but it wasn't until you fixed it and got the dyno readings that your upgrades really took shape - and there was tons of room to go wrong. Parts wore out too, so you were forced to buy new bits semi-regularly. Great stuff.
Later on they patched it so it gave you a preview of the new dyno reading, and I think that spoiled the process a little. I guess it made it more casual-friendly, right enough.
It'd be nice to hear how you can screw up your car here. That said, if all they mean is upgrading your turbo without also sticking on a bigger intercooler won't have much effect, that'll be disappointing.
In terms of the Storefront, it'll be good for the more casual designer. Folks like me, who don't want to spend weeks on a single skin, placing thousands of layers (did they not say a couple of thousand layers per section this time?) to make a pixel-perfect recreation of their favourite celeb will be able to share more simple designs. I could make a Martini logo which someone else then uses to recreate some of the classic paint jobs from the rallies of yesteryear. They spend less time on the fiddly parts, I don't need to make the full thing - everyone wins!