There are a few considerations and options in the selection of a knife: Fixed blade or folding, carbon or stainless steel, serrations, blade size & thickness, tang and handle, and blade profile and point.
Fixed blade or folding: Folding knives are far easier to carry, and having your tools at hand is vital. I highly recommend any person who is not prohibited from carrying a knife to do so daily, and a folder is perfect for that. However, for the heavier tasks that a "survival knife" may be called upon to perform, the only real option is a fixed blade.
Carbon or stainless blade: Either is entirely acceptable. A high-carbon steel blade will be harder, and will hold an edge longer than a stainless blade. However, a carbon blade requires more care than a stainless blade, or else it will rust (and chip and shatter.)
Serrations: Saw-tooth areas of a blade are great for cutting ropes and other fibrous materials. However, they're a nightmare to sharpen without special tools, and even with special tools, sharpening them can be a chore. Were I advising, I'd say don't bother with them.
Blade Size and Thickness: My folders are about the size of my palm. For a hard-working fixed blade, though, you want something a fair bit bigger, preferably twice that or more. You also want a thick spine (the back, smooth portion of the blade). 1/8" spines are a bare minimum, 3/16" or 1/4" makes for a far stronger blade, and allows for batoning (a trick I'll teach later, in another firemaking post.) Also, many knives have scallops for thumb traction on the spine. These are rather handy.
Tang and Handle: The tang is the portion of the blade material that extends into the handle. The longer the tang is, the stronger the handle attachment and the knife itself will be. For working knives like we're discussing, 3/4 or full-length tangs are all that really should be considered.
As to handles, the scales should be tough and durable, but provide good grip, particularly when wet or sweaty. A finger guard hilt is also a plus for preventing cuts from slips.
Blade Profile and Point: There are numerous options for profiles and points, ranging from traditional to radical. Some of the more radical designs show merit, but the tried-and-true profiles are that way for a reason. The primary profiles you're likely to encounter are as follows.
Any of those profiles make excellent all-around blades. The Clip Point profile can be a little more delicate, and the Tanto Point takes just a little more practice to sharpen well. The important point (heh) is that they all have a substantial amount of cutting area, a shape that can be readily sharpened, and a piercing tip sharp enough to make holes in wood or soft material.
As just one example of what a decent knife looks like (at a decent price, too!) the SOG Aura would be a good starting point. I know Erin has also been quite pleased with her Glock field knife, and it definitely meets all the requirements listed.
And in minor closing miscellany... There's a reason I linked to BladeHQ for this article. They're local to me, their service has been stellar, both online and storefront, and they're truly knife experts.
Also, some additional reading, if you want to get way too deep into blade and handle materials:
I'd really love to see everybody else's knives. Show and Tell ain't just for the schoolkids! Share em in the comments.