Just got the game a couple of days ago. Love it so far as it is but I am also looking forward to all the mods that are coming out. I got Oblivion long after it was released so there was a whole world of stuff to choose from. As a few have mentioned already what I appreciated most about my Oblivion experience and most likely will with Skyrim. It is the creativity and the ability to take an already great foundation and tailor the game to how I'd personally like to play. I've already found myself thinking while playing 'hope someone creates a mod for this or that' it would be cool. To me that ability to get mods and tailor is a plus for the game and entirety of the experience with the game over time.
I say time because with Oblivion it was not a game that I powered through and finished as quickly as possible. It was and still is (I occasionally revisit it) a game I played in between dozens of other games over a year or more. I think it's telling that Oblivion, even after a couple of years, is still sitting in my main game section on my shelf and not relegated to the collection shelf that gathers dust.
I played the game in a totally different manner then most if not all others. Usually I get a game and power through it. Sometimes finishing it within days. It then goes on the collection shelf. I also play MMOs (not Wow) which are also a totally different style. I have no desire to play, play, play right now and get to the end like I do with most other games and that's not a knock on the actual game. Something that would be with most other titles.
With Oblivion I ended up doing tons with more sandbox type mods. Spent hours building and growing gardens. (hope those mods are created). I got this cool armory mod with all the display rooms and spent hours getting and finding gear to fill it with. I wanted every display maniquin covered with complete sets of armor. I worked at creating and supplying my houses as realistically as possible. It was like a created my own quests and challenges using tools that came from the creativity of the community. None of which had anything to do with finishing it or 'progressing' my character in any other way then roleplaying it in my own imagination. When I got Oblivion I didn't have any plan to mod it or do all of these things. It just ended up that way. Heck it was even fun going through the dozens and dozens of mods and figuring out what to try. It was like I had a hand in creating my own game experience and wasn't forced to play it only as it was originally set.
For me all of this was and hopefully will be with Skyrim a plus. There's just no way the developer could or would make it and implement things that catered to me personally. I wouldn't expect them too. I mean I understand that many people would find growing a garden boring beyond belief and wonder why someone would spend so much time working on it.

With the ability to mod it though I could do that sort of thing just because I enjoy.
In fact one of the main reasons I got Skyrim was because of the expectation that it will be similar to Oblivion in that what I got in the box was not what I was relegated to play with forever. This is a positive in my books.
I will be playing it casually because I am enjoying it, while waiting for the mod explosion. I'm excited about that. For me it adds to the game experience. There's no doubt I would get my moneys worth of entertainment from just the vanilla game if I chose too. It's a great game. The creativity and personal tailoring that will hopefully come with mods make it a step up from a great game to another favorite that hopefully won't relegate it to the collection shelf in a month or less.