I've just completed Playthrough 2 with my Level 50 Lilith, and she's incredibly powerful, it not actually over-powered. There's one gun variant that should be nerfed in this game, it's not the HellFire SMGs, don't worry, I think it's the Double Anarchy ones. I got one of those SMGs, with a Level 26 SMG proficiency, it fires 4 bullets at 98 damage points per click, at medium but especially close range it deals more damage than the best legendary (orange) Shotguns and Combat Rifles I found, I simply "owned" the last boss in Playthrough 2 with it, and it took about 85% of my ammo count (amount at maximum, fully upgraded ammo quantity), firing for about a minute, while barely moving, the poor thing couldn't do anything.
Anyway... Playthrough 2 was much better overall than Playthrough 1, but the same problem in Playthrough 1 persists in Playthrough 2 as well, and that is missions (not regular mobs around, I'm talking about missions) being under-leveled after some point. In Playthrough 1, if a player does all the side-quests until the "Leaving Fyrestone" one, at which point he/she will be around Level 20 (if not more) then ALL the following quests in the game, I do mean ALL of them, will be of the "Trivial" and "Normal" difficulty, which means that about 90% of the remaining quests will be around one to up to four or even five levels UNDER yours, or if you're lucky the remaining 10% being at the same "Normal" level than yours. In other words there's basically no challenge (I repeat, I'm talking about missions here). The only occasional challenges will be found outside of the quests, in regular mobs when sometimes one or two tougher variants will spawn, like the big Spiderants and their "Overlord" variants, for instance.
In Playthrough 2 the exact same issue persists, again by the time you get to leave Fyrestone, which usually means that you're about Level 40 by that point, then all the following missions again are under-leveled, anywhere from one to about five levels weaker, and occasionally having the "Normal" difficulty missions. There's a big difficulty balance issue in this game in terms of mission difficulty. I have no problems with the mobs, it really is just about the missions. And since the missions are all Trivial or Normal their rewards are usually useless other than having the selling value if you do receive an item. I made a thread about that at the Gearbox forums and most of the posters agreed as well when they really noticed the facts, it's clearly there, black on white, and it cannot be denied, the missions after some point in the game offer absolutely no tangible challenges, it's as if the testers only had the time to test their characters up to level 20 or so and then they forgot about the rest, which is why the game IS sometimes tough at the beginning, both in Playthrough 1 and 2, when at first some missions have the red colored text and the difficulty indication being "Tough", "Hard" or even "Impossible" when they are from one to about three levels ABOVE yours. That system works until some point, after which it miserably fails.
But, other than that, it's fun, outside of the numerous other issues in the game. It's very sad though, because as of now, after finally reaching Level 50, completing all side-quests and the main quest and being stuck in Playthrough 2 with newly leveled foes at Level 51 with the new SuperBad and Bully variants I am finally getting decent and pretty much constant challenges. At this point about 90% of the enemies I meet are doing attacks I had never seen before, especially the Scythids and Spiderants, and the Bandits spawn with absolutely cruel gun variants that can cut through my 1395 points Shield with 146 recharge capacity, and that my only saving grace is my Mind Games skill when it kicks in. That kind of challenge should have been on my path at least occasionally in both Playthrough (and in Playthrough 2 before beating the last boss, before all foes re-level when that boss is beaten, I mean).