I'm thinking the things like mounted combat and the better looking underwater were held because they weren't perfected just yet, since there was no cost on them yet. But with as much seems to be added with Dawnguard, I don't see how they could have held that much back, I'm thinking they started working on the Dawnguard stuff before release, but it was too late/not even close to being done before release.
Indeed, altough I would prefer if they included it when they released the game. The fact that they added mounted combat and stuff via patches is good though, it's when you have to
pay for something that you would expect to be in the game when I start to get grumpy.
Dawnguard is good as well, it's a promising expansion, and I'm not sure if it would have fit in the final release or not. It's sorta like Shivering Isles or KOTN. And by fit I mean "it's more fitted as a DLC, rather than a part of the actual game".
I wasn't really saying they cut things from Skyrim, I was just saying that it would have been a more plausible explanation, than having limited memory in the disc.