I think one of the reasons that they made marking in 4E as limited as it is, was because of the serious pushback that got on the scope and complexity of 3E Attacks of Opportunity. Remember that hoopla? It didn't help any that the initial text on 3E AoO was a bit spread out, repeated apparently inconsistently in places, and lacking the full rationale of what it was modeling. Those of that got the gist of the idea upfront (often because of roots in other systems) typically had no problem with it, but plenty of other people screamed bloody murder. How much would marking be more readily accepted if the rationale had been made explicit instead of implied?
And none of that is taking into account rounding off corner cases and other simplifications for ease of play (with AoO, OA, or marking).
Which is just to show that on these kind of questions for a new twist to the mechanics, they are darned if they do, darned if they don't.