My point is we don't know that yet. It may be as amazing, big and persistent without the HDD, but it may not. I am taking a wait and see approach. The HDD may unlock some features of that game beyond faster loading times or seamless environments. For all we know, the game could suck either way!?!
I also wait to compare the X360 version the the PC version, as the PC version will be programmed from the start to make use of a HDD. I am curious to see if there will be any differences in the game fundamentally between the two platforms.