I've played it several different times, I often find that space in Stage one, area one feeling too claustrophobic. Especially near that bridge the cart rides over, I'm mostly talking about under it.
Stage Two has a few problems that people are moaning and groaning about when I play. The overhang before checkpoint one is an issue, since it seems that its the only place sentries have been built recently, every time. Actually, I've played a few games now where there are three or more sentries right there. Its one of the harder spots to stop the defending team. Perhaps another ramp up that leads up there, a little more opportunistic for the attacking team would help.
Stage Two, Area Two: The long mining tunnel is rather unused. There are two good reasons not to go there. One: Since we have to walk past the hole like ducks in a shooting gallery, there's no ammo crates and the only health pack, once you get over there, is the small one. I'd prefer that if you decide to change/add the packs that you place them so people see them right away. I'm talking about putting them right on the track against the wall, framed by the doorway.
Second: The door exiting the cart tunnel can be overpowered by one sentry gun across the battlefield. Any Defending Engineer who builds behind the rock where the medium health pack and Ammo crate remain, can have their sentry gun defeat the purpose of the tunnel. It was one of the hardest spots to take down the sentry from. The heavy couldn't, the ubered pyro was shot into the sky, and every time the door opened, the sniper was taking too much damage before he could try and take it out.
Personally, I don't like the one way door where it is. If it was a little forward, towards the point, using that metal frame instead, I'd be a little happier about it. That way it doesn't open by accident.
I haven't seen any problems with the other areas.
If you need any visuals for what I am talking about, I am willing to give.