I've gone through this same argument so many times that I might as well write my point down. I liked the instant kill system that was in Soul Calibur 4. Most players apparently didn't since it's not going to be in 5, what with getting replaced by very Street Fighter-y super meters. Lame.
The Soul Calibur series has never been the most popular fighting game among serious players. There's reasons for that, but I'm not going to go there. Still I do think that the instant kill system that was in Soul Calibur 4 was one of their best ideas. Had they put more effort into making rest of the game more seriously competitive and less effort into making boobs and breakable clothes, who knows? Then again, maybe boobs are the way to go, since at least the series is making enough money to keep the sequels coming. But seriously, let's not get distracted with the boobs.
Instant kill in a fighting game sounds like a bad concept until you actually know the mechanics behind them. Guilty Gear has had them for quite a while and in that series they are most often not worth it. In SC4 they also made them at least a bit tricky. In addition to the usual health gauge, characters also have a soul gauge. The gauge increases when doing active stuff like attacking and guard impacting and decreases when blocking attacks. The amount of soul gauge damage which is dealt when attacks are blocked varies from attack to attack. After enough damage the gauge will first start flashing red and after a couple more hits it's gauge break time. This is the instant kill window. Pressing all four buttons when opponent's soul gauge breaks ends the round right there with an instant kill animation.
If you read the last paragraph carefully you might've noticed what this system does to the game. That's right, it encourages active playing style, or rather, punishes sitting behind block. Most characters had attacks which did pretty heavy gauge damage, resulting in break in just six or seven hits. For some characters going for gauge break was actually a viable strategy. The system also made the game state more fluid: when your soul gauge is flashing red it's time to act. Respectively, if the opponent's gauge is flashing red it's time to bring out the soul crusher moves. The gauge actually did change the game quite a bit. Until Namco blew it. Turns out most players did not like the system, so they decreased the soul gauge damage of all moves in the game a lot.
There's always strong opinions about nerfing, but this time I really feel that the choice was utterly, inexcusably wrong. The number of instant kills in real play went from approximately 1-2 per match to 1-2 per twenty or more matches. In other words, insignificant. So Namco just chickened out and basically took out an entire mechanic from their game. The one mechanic that actually made the game different from its predecessors and also other fighting games. Now they're just going to replace it with something safer and I think that is really, really lame.
I guess I still end up trying SC5. The series is still the coolest fighting game series with weapons in it. I just wish they competed with Tekken and Virtua Fighter for quality rather than with Dead or Alive for boobs.