@Arkandel To add to Coin's reply, here are a few things to consider:
What are you using your bike for? Is it just for commuting?
If it's just commuting, then you might think of ignoring mountain bikes all together. The cheaper ones are heavy and slow, and the thick knobby tires are awful on the road.
If you like to ride on trails, then a mountain bike will be what you're looking at, but if it's still mostly commuting, I'd recommend getting slicks for it.
What are you riding your bike on? Do you have nice roads? Hills?
Where I live it's pretty flat, and I really only ride on the road. For my commute, I have a single speed road bike with caliper breaks that I got for $500. It's fantastic--fast, and easy maintenance. My only issue is that I'm heavy and my roads are like a warzone, so I find I'm fixing my tubes once or twice a month. It gets easy with practice
If you have hills, bump up for some gears. I got a second bike and the one or two hills that I encounter are now a laughable breeze.
If you have back/neck issues, many places will sell decent commuters without drop bars, or will even swap drop bars for risers.
If you're not into doing maintenance, I'd steer away from disc breaks. They're difficult to adjust properly and your wheels will pretty much never be true. If they're not perfect, you'll get drag off of them. Good for your quads, but might not be the most enjoyable bike ride .. My second bike has disc breaks and even my store's mechanic has difficulty getting them just right, in order to get rid of the drag, I have them set pretty loose.