My favorite reasons for playing Elder Scrolls is that I like the world that the game creators put you in. I like being able to do whatever i want, whether its completely ignoring the storyline to be the best Master Thief around, or just Plainly being the Most Destructive Dark Brotherhood Assassin there. The biggest reason i play this game is that i love wild encounters. I love jumping from a humongous bolder only to Find that i landed in the middle of Bandits
Why do you play this game?

1. Freedom. Like you say, Bethesda has created a world in which it is possible to do nearly anything one wants, to go nearly anywhere wants to go, when one wants to go there. As much as I respect Bioware, I cannot play their games. My characters tend to be an anarchic, anti-authoritarian bunch of iconoclasts who prefer to follow their own noses and get into their own trouble. They do not much like to be told what to do, most of them. No other game series allows me to play characters like these so completely.

2. Mods. Mods allow me to change the way the game operates according to the needs of each character. I can start a character out in the Arena, in a shipwreck, next to a fire out in the wilderness. I make houses for each of my characters that reflects that character in some way. I can add or subtract quests, gear, spells, increase or decrease difficulty; I can add new lands, villages, NPCs...whatever my character needs to tell her story, mods can probably provide it.

I wish there was an oblivion with a bigger world.. like Morrowind and Oblivion combined.. One of my favorite moments though is when you come off the boat in Morrowind.. where your basically off to do whatever the hell you went... it was much better than Waking up in a prison.. Morrowing kinda gave me a sense of.. If you wanna do something do it yourself.. That's why i hate Fast Travel.. it takes out the REAL enjoyment of the game.. lol we need a remake of Morrowing

Yet another reason to like mods: http://www.gamesas.com/index.php?/topic/1193087-tamriel-worldspace-modding-project/ Combine TWMP with http://www.gamesas.com/index.php?/topic/1093790-relz-sides-sailing-ships-version-09/ and your character can sail around the entirety of Tamriel.

I'll second the vote for 'Freedom'. thats my big draw. if I want to go left I go left. if I want to go over there and do something to that guy, then I go over there and do something to that guy (and maybe pay a fine for it, but, at least I got to do it!). If I want to be an axe wielding mage with the ability to pick locks then I can do that.

I also love collecting things in real life and in games. I swear if it weren't for Oblivion (and to some extent Diablo II) then I would be a hoarder in real life. In elder scrolls games I love to collect weapons and armor sets, display them, and pretend I own a museum. as a mage I am collecting all things magic; scrolls, staves, spell effects, soul gems, rare ingredients.

I love how random everything is, like seeing a dead person on the road and knowing you did not kill him. And that dead guy not being there when you play another character.

Do whatever you want
Pick whatever race you want
Play the game that you want to play
The character that you create is yours not some predetermined crap like other RPG's do
Dungeon Exploring and hell just exploring period
Random Items
Challenging fights
etc

Simply the open world sandbox freedom style of games that I have become spoiled with. I won't play any other style so readily anymore.

Freedom.