My opinion (since you are asking):
This makes perfect sense for creatures with animal intelligence. Not as much for normal (or higher) intelligence. If the PC's can use tactics, so should the badguys.
Also, I may have missed it, but did you consider a cavalier challenging his enemy? Thematically, that seems equivalent to causing a mark. It is directly provoking an opponent. However, does that replace an existing mark, supersede a new mark, or allow a creature to momentarily have 2 marks.
Finally, I can see PCs abusing this system. Imagine a high-damaging pc (like a wizard at the start of the day, fresh with spells) acting just 1 initiative step before a lower-damaging one. Under this system, an intelligent opponent could still be forced to ignore the person causing him/her the greatest pain, because someone else managed to hit him just before (s)he acted. Better yet, I could see PCs forcing an opponent to run a gauntlet of AOOs and flanks, just to get to the archer 30 ft away. An intelligent opponent wouldn't do that.