Experience has made the design team's TF designs better. That's only natural when working on three movies. Imagine if they had the chance, do you think that Optimus would look the same if they were to have designed him this the experience they have now? They know what they're doing now as opposed to 2007. Compare Let's say Ironhide (the messiest-looking Autobot) to Sentinel Prime who looks more whole and streamlined. Compare Megatron to Shouckwave and you can see what I'm talking about when I say that experience has improved the designs of the TFs while retaining the elements that made this aesthics unique.