for a bike to be a nice trail or off road bike it has to be relatively light & relatively strong & have a great shifting drivetrain to avoid miss shifts & stalls on transitions & climbs... most will say that shimano xtr is the best, with xt being a pretty close second, just a bit heavier... a bike doesn't have to be very expensive to be good but it can't be built from cheap crap either.
one of my all time favorite trail bikes was built on a trek 820 chromoly frame... this is treks cheapest bike at like just a few hundred bucks for a complete bike but of course that's with all junk parts included, but I had it built up with all best components, all xtr, chris king wheel set etc etc... came in right around 18.5 lbs which is light... the frame itself was only like 4.2 lbs which is only ~1.3 lb heavier than the most exotic light weights in my size...
one reason I loved that bike so much aside from the plush ride that only a chromoly hard tail gives, is that it just looked like a cheapo crap bike to any bike snob, yet I would out climb & just beat all the local hotshots with there mega buck yeti's, lightspeeds & canondales... that bike was the most fun, it was like beating a Ferrari with a honda

I always wanted to build a similar build with like a pacific frame or something, that would have really screwed with the other off roaders heads big time... the only time you seen a cheapo bike like that on the trails was with a n00b walking with it

some of those cheap toys r us bikes can be well over 30lb's & the wheels would bend out of true the first big hit & then the gears would just bind up... the components make the bike...
don't get too hung up on a fancy frame just pay attention to the components, but unfortunately good components can be expensive too, you just gotta balance bang for the buck.... there are less expensive stuff that is still pretty good... stuff like xt shifters, maybe a bontrager crank or wheel set or a simple but basic quality mavic rimmed xt hubbed wheel even, not the best but def not junk either... if you know components & you shop around & collect used stuff you should be able to built a bike for ~$500 or less that will rival bikes that cost 4-5 times that in the performance category at least... top priority is a good drivetrain/shifters & a strong wheelset, then everything else really is more about weight than anything else... it goes like this, : light, strong & cheap, pick any 2 :tongue (2):
the difference between an 18 lb hardtail & a 28 lb hardtail when it comes to off road riding is huge.... a 38lb+ full suspension toys r us bike is just about useless, maybe don't matter nearly as much for just a bike that's flat street ridden... there you can do with much less... for off road hardtail I'd say to shoot for no more than 21 lbs, don't get too hung up on heavier disc brakes too, especially the cheaper ones, they may look cool but they are anti performance when it really comes down to it in the long run...
oh & least thing, that's actually the first thing as far as importance, especially for off road is fit, make sure you get the right size frame, trying to ride the wrong sized frame off road is a definite recipe for failure when it comes to off road riding... fit should be placed above all else in selecting a bike, a great bike that is grossly miss sized will suck more than a crap bike that fits you in the long run...