Combos and breakers. The latter could be remedied by just making breakaway cause a hard knockdown, or by giving everybody a practical, universal armor breaker.
Dunno what they could do about combos in this one. Probably better to wait for another game than asking them to fix it.
I can't think of any mechanic I miss. Now that I think about it, there really hasn't been any mechanic characters. We've had ninjas, robots, army dudes and dudettes, gods, and cops to name a few. No mechanics. I guess you can't miss what you never had.