Freedom - total freedom within the world-spaces to do what YOU want. No forced quests (well, other than the starting dungeon). Open environments that let you go where you want.
Personally, I'm an explorer - I love worlds that let me go poking around, just to see what's out there. TES are wonderful for that.
For those who are into it, a rich lore background, where not everything is spelled out for you, and there's even contradictory information.
For the recent PC titles (Morrowind and Oblivion) the mod tools, which have allowed some talented people to extend the game way past what it was like out-of-the-box.
Nothing else I've found has come close, and I've been playing them since Area originally came out.
Freedom, endless opportunities. It may seem too much at first, but here's what TES does: it draws you in. You will find your place. The atmosphere of especially Morrowind is so amazing. There are more games out there with tons of stuff to do and choices to make, but these get boring because you don't have much reason to do something. The atmosphere of TES games is such that it creates a world you feel part of, want to be in, and it remains interesting.
And the mods... so many amazing things, you can customize the game in so many ways. If there is a small thing that annoys you, you can fix it most of the time without much effort.
The lore, books, depth of the story... is amazing. You can play the game, only read the dialogue presented to you, and finish it. Play it again, start doing research, and a whole new world opens to you. Conflicting accounts of events, metaphysics, religion, so many things are interesting and multi-layered. If you like depth, it is simply amazingly interesting to put some research into this. I spent many hours in game just hanging around in my character's house, reading the books I had collected.
There's just too many things to list here.
This is the kind of information I'm looking for. Why do you LOVE TES....not some much, you should go buy the game and find out. I will likely check these games out. I often pick up games to sit on for a while (like the Metal Gear Solid series) then come back once I have the time to play them.
So if you just found this topic or are coming back; in nitty gritty detail ..... WHAT DO YOU LOVE ABOUT THE TES SERIES?