I'm working on something right now, and I'm afraid of sticking too much stuff in the exterior cells, given that this could slow down someone's computer. (Mine is running fine, although the fan sped up once I started the game.)

Do you guys have an issue with too much stuff in exterior cells? What settings do you run Skyrim on?

You have to put a LOT of stuff in an exterior to make it a problem. It also depends on the exteriors size. If it's a full world (5x5 cells plus a bunch of LOD loaded at all times), you can't have as much. But if you're making an exterior that's secluded and empty except for the few cells in your enclosed area, you can jam pack it pretty dang full and get away with it.

I'm talking about the Tamriel Worldspace.

What upper limit would you suggest, on the amount of stuff?

I also am curious, I know I am way over the limit.

There isn't really just a number I can say... Think about it this way, a bit more than the cities. They are in seperate worlds so that consoles can run smoothly around them, due to their extreme detail (Compared to the rest of the world). So you can do a bit more than that, but shouldn't do much more without instancing it, or low end computers will have issues.

Lol what about 650/130 something? I only get lag in certain small areas however. However I do have many occlusion planes through walls and such so that not everything is rendered at once.