It's actually quite difficult to single out one feature as most important as I don't think any one is so important that is outweighs all others. However, a truly bad feature can drag down an otherwise decent laptop.
For me, I might say cooling is most important. But, I also wouldn't accept a laptop that sounds like a jet engine because of fans, or one that has abysmal performance to keep temperatures down.
So I guess that means the most important feature to me is the balancing of all other features.