Uhm...This is an hard argument because lot of times I have my mix that doesn't sound powerfull...or better, it doesn't sound powerfull as the mastered version. Punk mentioned the parallel and bus compressions but...I mean, when you compress the bass, the voice, the single drums parts, parallel compression on the drum buss, a little compression and saturation on the master bus...what do you have to compress? I try for days and days different compression on different bus but it doesn't sound powerfull like a great master maximizer. I'm not saying that this is the right way to follow, but it's my case/problem.
Anyway I don't think it's this big problem....I hear albums mixed and mastered by super famous producer that are totally squashed and iper compressed but no one seems worry about this.