You can probably do fine just buying something off the rack.  

Santa Cruz, Kona, Specialized.  Heh, most of the biggies make decent rides.

Does Felt make a mountain bike?  I haven't seen any around.  Maybe I need to get out more :-)

you are going to shop with. I have been doing a lot of looking in the 1750 - 2500 range and my best buy right now would be the Gary Fisher Hifi 29er. They changed the geometry of the bikes this year and after having tried out several back to back the GF has the most natural feel to me. The larger wheels are they way to go as well. Don't take my word for it though, get to a shop and ride as many different brands and models as you can before you decide. In my experience MTB's tend to have way more riding personality than road bikes and what might feel natural to me may feel cramped and twitchy to you. Kona, GF, & Specialized  all make some good bikes for hardtail/lower priced units as well. You can get a decent and reliable ride for under a grand for sure.

I don't have a ton of mountain bike buying experience, but I own a full suspension Santa Cruz Superlight and I absolutely love that bike. With the full suspension you can ride down virtually anything with confidence. FS has more stuff that can break and they're heavier, but I've had it for 6 years with no problems and I've ridden it pretty hard. It just eats up the trail when you descend and makes climbing trails simple.... Like a guy on bike snob said the other day "It climbs like a monkey wearing crampons and descends like a monkey wearing crampons dropped out of a helicopter"... A hardtail will certainly be lighter, but with mine, you can lock out the rear shock and turn it into a pseudo-hardtail if you want, which is nice if you have to climb on pavement and are a roadie (climbing on a bouncy FS bike will drive you nuts). So if weight is your thing, go hardtail. If you want the most versatile bike which you will be able to ride down trails you wouldn't hike down, get the lightest full suspension you can afford. No opinion about disk brakes. I don't have any and my bike stops just fine...

That's the most important question, I think.  If you're in, say, Kansas, I think we can rule out squishies with 6" of travel on both ends.  But if you're in an area with welcoming DH possibilities, you may find yourself unhappy with a hardtail.

After that comes:

1. does throwing yourself down a mountain sound fun?
2. any objections to used bikes?
3. whatcha feel like spending?
4. do you have any friends who have joined the rigid SS/fixie cult?  

I've got a couple of hardtails and a FS right now (in addition to the road bikes). If I had to have just one of them, I'd go with the FS.

~

Jimbo, jump on a bike with discs for a good run, and then reconsider :)  I had no idea what I was missing until I bought my Gary Fisher Sugar+ with BB7's.  Holy crap did it make a world's worth of difference (esp. the one-finger braking).

I really wanna build up a Santa Cruz Chameleon as a SS.  That would so rock.

The Kona explosif also has horiz drop-outs, and would make a pretty fun SS.  

LOL, so many bikes, so little time :-)

last winter as a way to get through the cold weather and to get out of my very rigid road mentality for a couple months a year.  My advice would be to pick up a used bike and ride it for a year. and then take the plunge next year into getting something new.  Personally, I wouldn't ride a hard tail around here.  A) it's too rocky, B) my achy back loves the suspension.  C) I don't plan on racing so who cares about a couple pounds if the trade off is more comfort and faster descents.  My bike is a late 90's Univega Dual Action Team which is an XC model with a fox rear shock and White Brothers SC70 (70mm of travel, but next time I'm looking at 100mm).  I tried a friends Specialized Epic with the "brain" which automatically looks out the rear when it's smooth.  very nice.  Next year I plan to go all-in and I'll be looking at Intense Spider, Specialized Epic, Santa Cruz or Rocky Mountain.  I'll stay with FS XC but look for a locking rear shock.  Currently I have V brakes which seem to be a major improvement of the Canti's from my first MTB 15 years ago, however I could see the allure of disc.  Plus, every wheelset still comes with a rim-brake option.  I am using SRAM grip shifters and XT derailleurs.  I like the grip shifters and will either go full SRAM X9 next year, or go with an XT/SRAM combo like I currently have.  
Unlike my road bikes which I built up, I may look for a complete bike and tweak some of the items afterwards.  MTB frames and components seem to be very expensive, but there are some complete bike deals which are very nice.  I love this one:
http://www.coloradocyclist.com/product/display/10530/

prefer full-sus for the rocks and gnarly trails they have to deal with.

Choices range from Mavericks, with Santa Cruz probably the most popular.  Our local national champ rides a trek fuel, and there's a healthy contingent of specializeds out there.  Don't overlook Haros, which provide a good ride for not so much spend.

Where I do most of my Mountain biking we have lots of hills, roots, and drops.  Hard tails work for all but a few of the rides, but you will be able to ride longer and harder with a full suspension.  I ride an Ellsworth Truth.  It is amazing.  The people I ride with sport Santa Cruz Blur, Yeti ASO, Rocky Mountain Element, Kona, Gary Fisher, and of course Treks Fuels.  So we are a Cross Country Crowd.

With Mountain bikes, unless you have a killer donor bike, you are always better buying complete in my opinion.

The key difference between rim brakes and disk brakes is the braking power when wet.  Being around Seattle I am often riding in the rain.  I currently have rim brakes and I have gotten myself in situations where I can't stop which is scary.  (Some my Christmas present to me is disk brakes and wheels)

My 2 cents - Hope it helps