Scanner resolution is also important, as in it's optical resolution. that's how well it can capture the full quality of your original. There's such a thing as interpolated resolution, but that's really only useful if you're enlarging stuff, it digitally adds in more DPI.
Bit Depth goes into this as well. Matter of fact, Bit depth may be the most important, that's how accurate it can scan. Go for at least a bit depth of 32 or 34, if you're aiming to scan really intricate stuff like photo negatives. Go for higher if really intricate color is important to you, but you don't need it if you're just scanning lineart.
Don't go for an all-in-one with a printer and fax/printer, they tend to have lower specs overall.
Light source is also something to consider. Older scanners (as well as higher end models) use cold cathode fluorescent lamps, and those give you really nice, bright, light but are more expensive, use more energy, and may require eventual replacement.
Newer scanners use white LEDs, which also have the advantage of not needing to warm up upon powering on.
Personal thing, not sure if it's just me, but I got less clean scans when I made the switch from an Epson Perfection 300 Photo (a CCRT model) to an Epson Perfection V500 Photo (White LEDs.)
It's like...the scans have a tendency to come out with naturally lower contrast, showing the details of rough pencil work, but also greying out my paper and showing a lot of the texture of a sheet of ordinary printer paper stock. Didn't have that problem before, but maybe it's my settings.