It can get grease stains and tenants aren't as careful as an owner.  

If its in ok condition I will sand, and stain myself as well.

So, the new tenant say that they didn't notice the stain on the paint and the carpet due to all the stuff in there. 

Either replacing them, or pressure washing them if they are concrete, or staining them with concrete stain if necessary.  

If tile is in good shape, while dated, get a dremel and clean it out and re-grout.There are "magic sticks" stains and wax to fill and cover nicks and scratches, try that.I wouldn't do much more than that to get it rent ready, if I were coming back in 6 years I certainly wouldn't update now to my desires and then turn 6 years of tenants on it!

Salvaged cabinets or other materials - you can re-stain or paint many old cabinets/furniture into a nice bath vanity that give it a look of class while being sustainable6.  

No more stained and crumbling drop ceiling panels from leaks.

clean everything with tsp and, following the directions, apply a coat of General Finishes gel stain in the same or slightly darker color.  

And this was a really heavy, long rain, so it actually seemed to leak worse and even stained the ceiling right in front of the window.

I recently used the Lowes version of grout sealer/stain.