Time sensitive quests/missions, as long as they're not TO time sensitive, are fine. But I don't really want the game as a whole on a clock. Because then you inevitably feel rushed and possibly frustrated as the first time thru a game you have no idea which side quests/missions will give you the XP/money/gear you need. Not all quests/missions should give you PhAt L3wt after all. The ones that do should be chained to other side quests.
Granny hires you to to stop the local street gang from littering her lawn with empty bottles and hypos. Having completed that the next door neighbor decides you're just the person to run the winos living in their garage off. In the course of that one of the derelicts offers you the pass codes to the office he use to clean in return for not chasing him and his buddies off. Now all you need to do is get past building security to access the secure computer in the office and play a few games that result in part of their bank account growing smaller as yours grows larger.
On the surface who cares if some gang banger is littering. But the chain of events can lead to a nice pay off.