Here's what I'd look for (in fact I am looking for a new scanner right now)

Look for the "dpi" numbers. If a scanner lists something like "up to
4800 dpi" look for the fine print. You should be looking for 2 numbers
(ex: 2400 by 4800 dpi, 4800 by 9600 dpi, etc.) The first number is the
number of dots the scan head can actually pick up: higher is better. The
second is the number of 'stops' it makes as it passes over the bed.
Higher is good here too but this is actually much less important than
that first number. (Ignore any numbers phrased as "up to ### dpi" or
"### resolution interpolated"--they're basically bogus software tricks
to produce big numbers for advertising purposes.)

Look for bundled OCR software: What's 'best' often varies based on the
kinds of documents you'll be scanning. OmniPage works well on modern
documents and is pretty user-friendly. I highly recommend Abbyy
FineReader which does the best OCR I've seen in a bundled package
(including non-English languages and older documents).

Consider buying PhotoShop Elements if you are planning to scan photos
and it doesn't come bundled with the scanner. It's much better than most
of the image programs that are bundled.

If you can demo some scanners, look at the interface. Does it allow you
to easily do multiple pages or do you have to click back and reset
everything for each page? Does it give you the flexibility to choose
your settings or does it just give you some generic dumbed down
settings? (HP is notorious for this, as well as being notorious for
seriously overpricing their scanners.)

If you want to get really technical, look at whether it uses a CIS or
CCD sensor. Here's a definition/comparison of that:

Right now I'm leaning toward Epson. The interface seems reasonably good,
the specs compare well for the price, they have many models that use the
CCD sensor which means they are bulkier than some but produce better
quality scans, and many of them are bundling Abbyy FineReader which is
definitely a point in their favor. (I already have PhotoShop products so
am less interested in what they are bundling for image editing.)

I've used:
  - Canon Lide 30 with PhotoShop Elements and OmniPage: decent
interface, good color balance, reeeaaally sloooowwww, good OCR on modern
docs, not so good on older docs
  - Visioneer scanners with PhotoShop Elements and TextBridge OCR: great
interface, decent color, mediocre speed, OCR was fine, only works with
Windows, not as long-lived as some others and sometimes have weird
install glitches. (Stay away from the models that put the buttons on the
horizontal edge--everytime you scan a book it presses something
  - HP: lousy/no bundle, lousy interface, slow, overpriced, may be
longer wearing than some but that just means you'll be frustrated for a
longer time (can you tell I don't like HP?)
  - MicroTek - this was a long time ago, but I remember the color was
great, speed was reasonable, OCR--well it was a long time ago so it was
an early version of OmniPage. Not bad.

- Hope