The main decision is whether you want a true laptop or a desktop replacement. If it really is going to be used on the move, on trains, on aircraft - all over the place - then you need a slim, light system. On the other hand if it's just going to be a desktop replacement that gets moved from time to time then who cares if it's a little clunky. If you're willing to accept a little robustness you can usually get a higher spec system for the same price as an ultra-slim.
At the extreme end of things a colleague just bought very chunky new Toshiba which has a desktop P4 processor squeezed inside (it was brought to market before the mobile P4 was introduced). Anyway, it was a lot cheaper than a slim system with a mobile P4 - plus it generates enough heat to warm a small room
Myself I have a Dell Latitude C600 - quite slim and light - and I regret not having got something a bit larger like an Inspiron because mostly it just sits on my desk at home. Ok, it's less than 2.5kg but once it's in its bag with the power supply, a mouse, some CDs, papers etc - would I notice the difference if it weighed 3.8kg? Unlikely.