I spend literally over 300 hours in vanilla Oblivion when it first came out back in 2006. Yet when Fallout 3 dropped in 2008, with vastly improved gameplay mechanics than Oblivion, I only put about 20 hours into it before stopping. NV came out next, featuring some very enticing gameplay, and yet I stopped playing at roughly 30 hours...
Did I enjoy both Fallout games? Heck yes I did!! I was blown away by FO3's intro, quests, etc. NV astounded me with some awesome things like crafting, character needs, and great dialogue.
And then we get to Skyrim, and I easily sink 40 hours into it the first week, with no end in sight.
I think for me, the atmosphere and aesthetics bear a huge more amount than I realize on the amount of time I desire to spend in a world. Oblivion's sugar sweet fantasy land and Skyrim's beautiful, mountainous landscapes are far easier to "live" in than Fallout's grimy, dingy wasteland.
When I'm in a tavern or just taking a stroll in TES, I pause to look at the sights, or spend time hanging around areas simply for the beauty and feeling I get from it. In Fallout, I'm in a rush to get from one place to another, or to get out of a building because everything in it is so dingy and dirty. In my eyes that's a sign of success for the aesthetic feel that each series goes for, but does it affect your playtime and desire to spend time with it like it does for me?