There's virtually nowhere in the world where something that goes faster then 32 on it's own power is thought of as being an ebike... Still I'll assume you're staying in bike speeds, say to ~40km/h or so... In this case pretty much any bike will do. The type of bike you choose really depends on where you live and the bike's intended use. I get the feeling lots of people chose MTB's because they're still currently trendy, though in my neck of the woods this fad is getting phased out, replaced by road bikes... But hey, when fashion and functionality conflicts, I prefer functionality...
I'll use MTB's for the following description, because that's what Wall-Mart has as their highest end model, at 299$... Although they now carry at least one road bike like mechanism, GMC Denali... For starters, if you plan on anything other then occasional usage, I'd strongly caution against anything that looks like it came out of a department store, because it won't be of good service. For example, in my neck of the woods 300$ will get you a dual suspension bike at Wall-Mart. It will be a bitch to ride uphill due to weighing a ton and shitty suspension, and hairy to ride downhill due to shoddy brakes and weird noises. Plus it will almost certainly have problems within 500kms. Now the same 300$ in a bike shop will get you a new, basic but solid and (by comparison) very light front suspension bike with substantially better components. Or for the same 300$ you can get a good used dual suspension bike, with quite good components, though older ones. So long as the bike doesn't have obvious rust or signs of having gotten heavy use, or lack of maintenance, then these can be a good deal.
Otherwise, a bike that doesn't fit you is a bad deal no matter how cheap it is, because it'll be uncomfortable and will cause injuries in the long run. I like this fit calculator, having used the values it gave me for "french fit", yielded a superbly comfortable ride. That is, the bike is optimized for comfortable riding rather then max power/aero. It seems to match me to a T, but even then once your bike is put together you'll probably have to do some more minor adjustments before it fits you flawlessly.
http://www.competitivecyclist.com/za/CCY?PAGE=FIT_CALCULATOR_INTRO
If you're looking at an MTB frame, make sure it's got at least posts that stick out on each side of the fork for brakes, or a disk brake mount near the axle. Older MTB's, even fairly high end ones, sometimes have side pull calipers that are held on the bike through a hole in the middle top of the fork, but these calipers are kinda scaled up version of road bike brakes, and they bend terribly under use, not very good. If you're looking at a road bike though, this type of brake is more then adequate. Avoid bikes with the rear brake under the chainstay, otherwise you'll be stuck with a rear brake that rarely works. These haven't equipped mtb's in years, though. Also if you're buying a new bike from a bike shop, then you can pretty much count on the brakes to be more then adequate. From wall-mart, may as well factor new brakes, pads, cables & levers. If not right out the door, it won't be very long... Avoid things with plastic (except if it's just decorative) on the brakes or levers, this will bend and give a mushy feeling. Also avoid steel rims if the bike has rim brakes, because you'll stop faster pulling a Fred Flinstone when it's wet...
Steel generally make for frames that give a nicer ride then aluminum ones, but not always. Cro-mo steel would be preferable, and a frame made of butted tubes would be somewhat lighter then one made of straight gauge. Aluminum frames tend to be a little lighter, but they're also easier to break. If you care for them, make sure to look for fender & rack mounts near the fork ends, in the middle of the front fork and on the seatstays. Also check for clearance for wide tires if you want them, the chainstays especially tend to be too narrow on most bikes.
A steel fork that's curved is suspension, much nicer on your wrists then a straight one. But good grips or bar tape with gloves may be all you need, though it won't cut the mustard off road, a telescopic fork is a real plus in the rough. But unless you plan to throw yourself off cliffs and take multi-meter jumps on a regular basis, full suspension is probably overkill. I've been thinking of replacing the rear shock on mine with a block of wood or a pipe or something, as I've been using it pretty much exclusively on-road, It's even got semi-slicks on it at this point... Suspension will also rob a lot of power, on the rear especially. Asides from full suspension, there's also soft tails, like Xyster's bike. It's rear suspension consists of a short stout shock in the seatstays, there's not much travel but such bikes are said to help for climbing steep lumpy hills. You probably won't need suspension to ride on the road unless they're outstandingly bad, otherwise running fat tires gives suspension while keeping a lighter bike with lower maintenance. Again, make sure there's clearance to fit fatter tires if you want them.
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