The appeal is the fact that you can do pretty much anything and go pretty much anywhere. This is something that the Elder Scrolls games have always pulled off really well. I've noticed that people tend to either really love it or really hate it, so there's no real guarantee on what any individual will think of the series. On top of that, each installment has its die-hard fans and its die-hard haters. There's quite a few who maintain that Daggerfall was the last good one.
Personally, I love them all and there is no release that could possibly excite me more than Skyrim.
Anyway, besides the freedom, there's the abundance of quests and locations, and the potential for a completely different experience each time you play, due to using a different character build. That's something that was a little lacking in Oblivion, but I still got tons of play time out of it.
They're generally not that well-written, but they're not...awful, either. The characters are mostly pretty uninteresting, but the factions, politics and history of Tamriel can be pretty engrossing, if that's your thing. I think the Argonians, Dark Elves, and Dark Brotherhood in particular have fascinating histories.
Combat's usually only passable, and stealth has yet to work really well. It looks like Skyrim will be far and away the strongest installment in this department.
Really, you have to try these games for yourself. There's not much they can be compared to, and they certainly don't appeal to everyone.
