Okay, I will bounce in here with a topic that NOBODY has actually put forth: POWER SUPPLY. There has been a lot of technical discussion above about how and why and if an amp can double the power on half the resistance. In my opinion, this idea of doubling power really doesn't matter. The major factor of these larger amps is the power supply. This is one of the primary reasons why people buy 500 watt or 1,000 watt monoblocks. The power supply (i.e. transformer and capacitor bank) are large enough where it does not allow a radical drop in voltage when music transients hit. This can be a large drop or even a tiny drop/fluctuation in voltage (even on somewhat quite music). With a large power supply, you will have smoother and more constant available voltage when the transistor circuits pull current to power the speaker. The amount of watts produced is almost a meaningless number because you realistically won't ever use more than 30-50 watts anyways. The power supply in larger amps is a large reason why the larger amps sound better, less harsh, smoother/fuller response, stronger bass, no brightness/clipping, more refined sound, etc.
The difference between the 400 watt monoblock and a 500 watt monoblock is actually very little. However, the difference between a 100 watt and 400 watt is pretty significant.
The other factor is whether the amp will be stable at 4 ohms or even 2 ohms (as some speakers definitely do drop down to 2-3 ohms in the bass area). This is the other area where power supply has a significant difference.
Obviously, different amps are voiced differently even though they are sized the same.