I want to build a bigger workbench than the one I have now, and in my basement are several old doors (flat hollow-core doors, no panels or windows or anything) left by the previous owner. My idea was to take off the doorknobs and mount the door on a frame made of 2*4s; I'm figuring the door will be flat and stable. Any reason not to do this?
Hollow-core doors would not make a very good workbench top. The surface layer is very thin, either 1/8" material like a soft skin Masonite material. Very easy to drive your fist through it. Would not take hammer blows, pounding, etc.
I did purchase two hollow-core doors to lay on portable saw horses. I can lay plywood on the doors and use the gap between the two doors as the opening for the saw blade. This provide full support for the plywood sheet on both side of the cut. Doors are light, easy to set up and put away.
The only reason to not do this is that the surface skin may not significant amounts of abuse before it's too torn up to be usable.
If you need a bench top that can take some abuse, the door will work as an excellent base for your bench top. Frame it in 2x4 (you may need a center rib, too - that will be best determined through experience), then add a top layer of 1/2" or 3/4" plywood or OSB (depending on budget and desires). Screw this layer down through the edges of the door (where there's solid wood) and into the 2x4 below. Ensure you counter sink your screw heads to you don't hit them with tools (especially plane blades).
The ply/OSB will take the abuse while the door itself will ensure you've got a nice flat surface. The screws will allow you to easily replace the work surface in the future if this one gets too chewed up for your liking.