Personally, I'd be inclined to say the biggest problem for me is that the Eco-Motion engine hasn't stamped out a lot of legacy issues. Several years and a new engine later, we're still dealing with canned moments, players getting stuck to each other and feeling like they're running in mud (though the latter has gotten a bit better), cheesy interceptions by the AI, and so on. It might sound like I'm cheating by naming multiple problems, but I think they all come under the one umbrella of "stuff that's needed to be fixed for a while, and hasn't been". It's not that there haven't been any improvements or nice additions, it's that there's a laundry list of problems with the core gameplay, and many of those issues have persisted for several releases now.
To put it another way, after maintaining its high quality for many years, the game is starting to show its age. Problems we were once more willing to overlook and be patient about, owing to our general satisfaction and the fact no game can be perfect, now lead us to ask "Really? There's nothing that can be done about that?"