Whatever else the games faults may be, this I think is a positive thing. In this respect, V requires more forethought and planning. You really shouldn't be able to turn your civilization on a dime, 3/4 of the way through its history. This accentuates the empire building aspect of the game, in my opinion.
From a player's perspective: Locking the player into a win condition at the very start of the game and then not giving them any option to change to a different win condition is not fun. You're forcing the player to:
- Decide ahead of time what win condition they find fun. That leaves no room for players to explore the different win conditions. They might get 1/2 through a game and find that they're having a lot more fun chasing a cultural win or find that instead of a culture win, going for the space win might be more fun. In a game as rigidly designed as Civ5 - you don't have that option unless you've already run away with the game.
- Forcing the player to make a major game decision at a time when they don't know who the players are, what resources are available, what the geography looks like, or what the major power blocs are. In Civ4 - you could keep your options open until all pretty much all the land was claimed (or about 1/3 to 1/2 into the game). You didn't have to commit whole hog to chasing one social policy at the exclusion of another.
So, if you guess wrong at the start, you're either doomed to a long draw-out loss, or you restart the map. For the empire-builder types, neither choice qualifies as fun. The fun of being an empire-builder is (a) exploration (b) dealing with the neighbors (c) grabbing land (d) growing the empire (e) figuring out a win condition. You get emotionally invested in the map and the locations of the dragons in the margins.
Good games don't force the player to make big decisions early on. Yes, you're taking gambles early on (settle here, don't settle there, what to build), but nothing there that forces you onto a particular path forever and ever.
Which is my chief complaint with social policies. Yes, I agree that there should be a stepped ladder/tree where one policy unlocks more advanced policies. That, at least, is an interesting design decision. What I don't agree with is that you can never change government types. You should be allowed to change governments, just like you did in Civ4, but the social policies that you have chosen before should merely add flavor/bonuses to those governance types. You should do best if you stick with the government types that most closely match your social policy picks, but if plans change, you should be able to suffer along under a different government type (with reduced effects/bonuses or without the bonuses from your already chosen picks).
Right now - decide at the start of the game whether you want a small empire or a large empire, because you can't change that choice later. Decide at the start of the game whether you want to chase a culture / economic / science / military win - because you won't be able to choose differently later. Choose whether you will oppress your citizens or give them freedoms, because you can't change later on.