"Can" not "must." I don't think I've ever chosen to do every faction on a single character, but I'm I'm fine with it being me rather than Bethesda who decides which ones I pursue simultaneously. In fact, this one prefers it that way. If some prefer to exhaust every quest line in a single playthrough, that's their choice. If I prefer not to, that's mine. I think there's a fundamental conflict between folks who want the game to provide more limits/direction and those who love the freedom of TES games.
Someone rightly said on an earlier post (was it this thread?);
Freedom needs to always have limits. Why not just give us an unlimited supply of money? We don't
have to spend it. If we want to be poor, simply don't spend it all. There's many other situations like this. Freedom needs to be managed correctly to stop the game being far too easy, stopping unbelievability, and to make the game enjoyable.
Also, the fact that I
CAN join any guild I want really breaks immersion. The people aren't believable, as they don't seem to care about thier guild.
There where actually so many opportunities to fix this in Oblivion. I could have joined Blackwood instead of the Fighter's Guild. I could have join the necromancers instead of the Mage's Guild. I could have joined the Imperial Legion instead of the Thieves Guild. But no, Bethesda decided that, instead, I was to be limited to one morality, and one morality only. I could never join up with the Legion because I believe in the law, or join up with the necromancers, because I think they're unfairly treated. It's completely wrong. There should be various paths that my character can follow, and each one should close a different path.
Also, about the DF2, MW2 and OB2 thing. Why shouldn't we? If I enjoyed Morrowind, I buy the next game because I want to get that same bit of enjoyment. I don't think "oh, look, the next game in the series. That must be completely different, considering it's the same genre, setting and series."