Best advice I can give: be honest with yourself about how you are planning to use it. If you plan on trying out some downhill, get a fully suspended - thats all they are good for cause they are too heavy and generally need more energy to ride. If your looking to do some trail or single track stuff get a light fully suspended or a hard tail. If your a commuter get a 29er or a hybrid. If your into long street rides get a road bike.
Alloy bikes almost require suspension because they are stiff. A good steel frame is more supple, but if your heavy they can feel a noodly in the corners. Titanium is even more noodly. Carbon fibre bikes are incredibly stiff, but sing beautifully at high speed - they really do make noise, at least mine did.
Most fully suspended bikes cant carry paniers.
Get disc brakes - they resist dirt and moisture better than any v-brakes or caliper brakes.
Fat tires require more energy to spin than skinny
Dont buy a used bike unless your sure its a cream puff. By the time your done repairing a used bike you will have spent the same amount of cash as a new bike and at least with a new bike you get a warranty and possibly the odd free tune up.
Buy good gloves and a good helmet these are more important than the bike