I'll get right to it. I've played (almost exclusively) for the last 8 months (I was late to the party, but I played a lot of T6 before I got TTT2).
The instant replays are not fun to look at. From my understanding, this was done in an update, so as to not have your tag-mate pop in from out of nowhere. I would rather have that than the current replays. When I go back and play T6, some of the KO replays are very satisfying to watch--specially if you land a tricky/risky combo. As it is right now, I'd rather have no replay to save time than the uninteresting system they have currently.
Some stages are way too tight. This is infuriating because the wall carries in TTT2 are crazy extensive and the claustrophobia is real. Which leads me to number three.
The walls have too much magnetic abilities. I know you've had this experience: You get hit, you fly back properly--you wall splat when it doesn't seem like you should have. It's been going on since T4 (introduction of walls), where any light push into the wall makes you wall splat full-damage. It would be better if there were different levels of contact (light damage/snap animation for showing proximity, medium damage/obvious dent in the wall for incomplete contact, and the usual wall splat/break for full contact. This leads me to number four.
Why isn't there a system to let you know proximity to the wall behind you? Something simple like a vertical line flash on your side of the screen would be non-invasive, sufficient, and ramp up excitement/anxiety in a match. Too many times I thought I had plenty of space behind me, only to wall-splat.
The blue life bar needs to be easily picked up by the corner of your eye, like the orange bars in Tekken 5. In some stages, the blue blends into the background (blue skies or bright tops), and you have to directly look at the bar to understand how much life you have left.
They need to make it more obvious which life bars are yours 100% of the time. Matches get crazy, and you'll switch sides--sometimes too often. This can lead to confusing situations where I think my life bar is the bigger one--and then, I find out in my KO that I was the shorter life bar. Your name flashing, or a simple YOU in a small box, next to your name would be fine. For local play, P1, P2 in blue/red boxes next to names, with small blue/red triangles above each player would fine.
You guys remember the sloped stages of T4? TTT2 has a minor version of that. Two stages in particular make the battles hard to see--not to mention, strain-inducing on the eyes. I love how Tekken pushes the boundaries on what it can do with each game. However, there are some things that are super bad. Uknown's Second Stage gets really bad when you fall annd get covered in black gunk that blends with black everything of the stage. Coastline Sunset gets "dark" and "shadowy" when the sun is in the background. While I appreciate the true-to-life feel, it detracts from the gameplay. And most of the time, anything that detracts from gameplay, is bad.
Customization Needs a Z-axis. So many times, there's a simple customization thing I want to do--and I have to either give up, or mess around with the current system for way toooo long. I love customization. A big part of my fun is making new costumes. They've made it better than T6's system--but there are still times where it makes me want to break my control. For some this may be a stupid complaint (I've seen the circlejerk of I hate costumes around here, so I get it--it's not for everyone--but some of us really love that aspect of the games).
Those are my complaints. Most of them would be minor fixings--I don't know if T7 has changed any of these as I'm kind of keeping off news for the most part: I want to play the game as cold as possible when it comes out--as this would keep my expectations in check. I've seen the game reveals and very little gameplay.
Mostly, these are just things that grind my gears--I don't think they make the game unplayable.