Just checked the box: Oblivion is rated 16+, but FO3 is rated 18+.
Off course these are EU ratings outside the UK, so they might be different from country to country.
That of course depends on which rating system you use. The ESRB rating for Oblivion is also M. So an M rating is hardly anything new to the Elder Scrolls. Although Oblivion was rated T upon release, with the rating being changed later on due to reasons I don't want to debate again, though this MAY indicate that Skyrim's mature content is stronger than Oblivions, and therefore was harder to miss, or it may just be that ESRB examined Skyrim more closely to avoid repeating the situation with Oblivion, as I'd imagine needing to change a game's rating is something they'd prefer to avoid. Honestly, though, I'd say Oblivion only barely deserved its M rating, I wouldn't go as far as to say it should have stayed rated T, but it's definitely pretty tame as far as M rated games go, no strong language, and fairly tame in terms of sixual content, now there's some blood, but it fades pretty quickly, and the only gore the game has is in the environment and looks pretty fake even by the standards of other 2006 games, cut down on the severed heads on poles and hanging partially decomposed corpses, and I'm sure the game could have kept its T rating.
But anyway, the M rating in itself doesn't mean much to me. The ESRB rating is not an indication of quality, just of what age range ESRB deems the game appropriate for, and the same is true for all age rating systems. Though it is true that I was sort of hoping Skyrim would be rated M, not because the M rating in itself means anything, but rather because some things I want from Skyrim would naturally earn it that rating, but the M rating doesn't automatically mean the game has everything I want from it. So I'll judge if I'm satisfied with it only after I've seen what we got. If it's rated M because there happens to be one topless woman in the game, but the rest of the game would not warrant anything higher than a T rating, I can't say that would be what I want.
Lots of people DO want another Thief game. Just because it's not what YOU want doesn't mean it's not what PEOPLE want
Of course, it's also possible to want another Thief game, but not want Skyrim to be it. Some people might want the next Thief game to be called Thief, not the Elder Scrolls.