The kind that you see in WoW and similar games. You have tons of hotkey bars for skills, you usually use tab to cycle through enemies, and its mostly about cycling through skills in a certain, efficient, order, depending on the situation. Usually comes with the "holy trinity" package (tank / dps / healer) and quest based leveling in the form of "kill x monsters".
Ive tried a couple of games that used tab targetting, could never get into them. I found it a huge snore fest. Unless im playing the healer, I can setup a macro to do my skill rotation and just watch 95% of the time, while occasionally using pots or moving my character to the next enemy. I don't feel like im actually playing the game...I just feel like im playing a rhythm game and hitting hotkeys in the correct order.
But obviously, tab targetting is very, very popular. Although I think that just might be people "settling" for tab targetting because they like other aspects of the game.
Is it just because WoW popularized it and most people haven't tried any other combat systems that are significantly different?
I remember all the BDO hype, that it was going to be a WoW killer, etc. Obviously it never got that far, but BDO does seem to be fairly popular while having a combat system that is somewhat better than WoW. Maybe instead of "WoW clones" we are going to see a lot of "BDO clones" popup in the next couple of years and it will encourage more MMOs with a more interactive combat system...