I probably would've had the same reaction. I haven't bought an rMB, but I have been using a 4K iMac recently (I bought it off my friend) and it's decent, but I've noticed it's a little sluggish and not "zippy" at all. And I think a lot of that has to do with the integrated graphics (it does have SSD). Especially when I use my Windows desktop afterward, it feels 50x faster.
So I don't know, I really hope that the combination of good processor, SSD, and a decent dGPU allows the new MBP (15") to be quite fast and powerful. OS X is such a good OS. It deserves a powerful machine.