If I recall correctly the big thing with dx10 cards is that there are no longer parts of the gpu that does only vertex or only pixel processing. Everything is now general use so the card can use all pipelines for any process, thus improving efficiency. Tis can't so I am not 100% so forgive me if I am wrong.
GL features an extension system, that allows hardware vendors to add new stuff themselves. This is great to get new features to play with, but not so good if ati/nvidia end up doing similar with different extensions, or that certain features arent wide-spread supported. Especially when it came to rendering-to-texture…