Oblivion was my introduction to a huge open world of forests and mountains and valleys and rivers, fantasy cities and villages, towns and settlements. Caves and dungeons, ravines and cliffs!
It was the first game that actually made me say, "wow, is this real?"
'Course, once I actually started playing the game bit and got past how pretty I thought the graphics were at the time, I noticed that it was somewhat lacking immersion (for lack of a better word--I mean that occasionally I actually felt like I -was- an Orcish warrior traveling across the land, but 99.9% of the time I felt I was just playing a video game).
I wouldn't herald the Elder Scrolls as being epic and god-like in proportion, but I will say it took a general concept of "huge open world that you explore and can interact with in various ways while having character customization, items, fully-interactive items in the environment that you can move and place in whatever way you want, spells, swords and axes, bows and shields, leveling, dragons, gods and demons, and all that other fantasy stuff," and for the most part, delivered. The only problem is that it delivered it without packing peanuts in a box that had been bounced around the moving van a few times and suffered some dents and scratches--and when you opened the box up, you realize how messy and disorganized everything became during the shipment--and while you can occasionally reorganize stuff and piece together the broken bits with Hot-Modification-Glue and All Purpose-Sealant, the fact that it arrived in such a state keeps getting in the way of over all enjoyment.
...err, I went way too deep into that--so, my feelings: Oblivion was good not great, Skyrim is going to be alright, and I'm most definitely going to buy Saints Row: The Third. Just had to throw that in there.