Personally, I'd like to see TS4 use rabbitholes only when there is no other feasible way to include something in game. I'd actually like to see my sims at work or at school and I'd like to see pretty much every job have it's own gameplay elements and play some kind of role in the community. Here's a few examples of how different jobs can be more meaningful:
Law Enforcement: Police officers could patrol the neighborhood, investigate disturbances, and arrest trouble-making sims.
Military: Soldiers could protect the town from alien invasions. I know this idea seems a little out there but the only other thing I could think of for the military career was to provide aid after natural disasters like the National Guard would, but everyone's against natural disasters.
Music: Instead of a music career, I'd prefer that we got the ability to put together bands from the start, with a few elements from Showtime mixed in. Also, bands need to be considerably easier to set up. In my game, everyone keeps wanting to play the same instrument.
Crime: Criminals shouldn't have an actual career; it kinda defeats the purpose. Instead, sims of poor moral fiber should be able to do various illegal activities, such as breaking into homes or places of business.
Politics: Sims that follow the political career could be elected mayor and pass laws that actually effect gameplay.
As for the whole rabbitholes vs. seeing your sims at work debate, I think I may have compromise. I get how some people are concerned about how showing sims at work could eat up a lot of processing power, while others just don't want to see their sims at work or school. So, my idea is to make workplaces into a type of pseudo-rabbithole. When sims go to work, they would disappear into the building in which they work and their entire workday would be simulated as it is in TS3. If you wanted to take more direct control of their job, however, you could click on the building and select to open it up. This will allow you to watch your sim at work or take control and instruct him to tackle certain projects, chat with coworkers, ask the boss for a raise or a promotion (assuming your sim has worked hard enough to deserve one), or just simply slack off. Should you get tired of following your sim at work, or just want to see what another sim is up to, you could click anywhere on the lot, select "close lot" or whatever, and the lot will revert back to a rabbithole.
The only problem this still leaves, however, is the fact that many rabbitholes aren't quite to scale. Hopefully Maxis will think of something in that regard. Maybe TS4 will allow larger lot sizes.
