Quality is a big one, but even many good small mixers (Mackie Onyx, etc.) have very good, clean pres if you just want an "accurate" sound, especially from typical condensors. Some mics (ribbons, some dynamics, etc.) are better with other pres designed for them; this is more about "matching" than overall "quality", but it makes a difference.
Some pres impart a coloration, and people buy them for this. Tubes are big here. Cheap tube pres are actually solid state mic pres with a cheap tube circuit stuck on (basically a fuzz box); some like this effect, but it isn't the classic "tube warmth" they advertise. Others use the tube in the main circuit and are the "holy grail" of pres, like Avalon, Manley, even the old Peavey VMP-1.
Go listen to a high end pre to see if it matters to you.