I don't care about colour, so I want a mono laser printer.
I simply won't tolerate a printer without duplex.
The paper tray (all one of them!) has to hold a whole ream of paper.
Toner cartridges need to be cheap, high capacity, and readily available. Preferably from cartridge refillers.
The interface has to be non-existent and/or not suck.
It needs to be networked. Obviously. Wifi would be nice too.
Postscript would be nice, but I can live without it. Mac support and bonjour/zeroconfig are necessary.
I'd like to not have to go scrabbling for the driver CD whenever a visitor wants to print to it. So drivers should be readily available online: a straight link off the first google hit for "manufacturer model driver". I don't want to navigate through a dozen pages and dropdowns.
The driver shouldn't install a shedload of crap on my computer. I just want the driver. And I don't want it to offer to install acrobat. Not even version 5.
Cheap would be nice. Reliability is also vital, but that's something you only find out about after a couple of years. I'd like it to Just Work.