On what basis are bikes categorised? I am sick of hearing guys, categorising their MTB as per the money they put on to it. Is it really that much, or their is something more like stronger, more aerodynamically built. So far, I have found expensive bikes to be more lighter and durable.

But still the question remains, what are the factors that affect more when categorising a bike. By categorising, I dont mean MTB, Road Bike, Down hill bikes. I mean what some refer to as as High Level MTB, Mid-Level MTB or some as Tier-3 Bikes or Tier-2 Bikes.
Weight and durability are indeed factors, but it's also about the performance of components. The "high level" bikes are for a "high level" of performance. They're usually designed with competition in mind. 

The shifting has to be perfect, the brakes need just the right amount of power, the suspension has all kinds of adjustments, tires specific to the terrain on that day are used, etc.

Of course, Average Wealthy Joe will buy one of those bikes and brag about it, but hey, be happy he's financing your favourite bike companies. ;)