Borderlands 2 doesn't let you choose from difficulty levels. Players are expected to deal with the game's various difficulty spikes by teaming up online, or grinding and soldiering through. This limitation is what makes the new Mechromancer class stand out as such an interesting addition: Gaige's skill trees replicate a more traditional difficulty scale.
Gearbox lead designer John Hemingway created a stir when he referred to one of the skill trees as "girlfriend mode." But indelicate phrasing aside, he was right that the skill tree seems custom-tailored to serve as an "Easy" mode. What Hemingway didn't mention was that the other two skill trees function as "Medium" and "Hard" modes, respectively.
"Best Friends Forever" comes equipped with skills, stat boosts, and abilities that are actually a bit overpowered compared to the other classes. This felt more akin to the first game, wherein certain classes could be made into wrecking balls. Gaige has that feel from the start, that one player can easily lay waste to a massive field of enemies. While it's an odd choice for the imbalance with other classes, it recaptures the feeling of empowerment from the original Borderlands. You could choose upgrades from this tree at random and still probably exit the menu a pint-sized murder machine.
The "Ordered Chaos" tree is just the opposite. It offers a series of complex trade-offs, requiring detailed stat-tailoring that would probably turn off some players. The "Anarchy" skill is its centerpiece, which itself sacrifices accuracy for damage with every stack earned.
The middle-tier, "LittleBigTrouble," is a more reasonable alternative with an emphasis on elemental damage. It's not under- or overpowered, but a nice way to experience the new Deathtrap ability.
Having already played through the game for review and therefore used to a mid-level challenge, I dabbled in both extremes. I experimented with the Chaos tree first, which made the robot companion Deathtrap more of an aggro-target than much of a help in battle. He was tasked with distraction while I usually relied on a trusty shotgun--because if I'm getting increased damage for lowered accuracy, I might as well use a weapon that requires me close the distance anyway. When I respec'ed to the BFF tree, Deathtrap was a better tool to do my fighting for me while I stood back with long-range weapons and picked off stragglers.
That said, even early in the game it's easy to see how Deathtrap can break some encounters. He made short work of Boom Bewm almost single-handedly. When I came in contact with Captain Flynn, Deathtrap whittled him down to a fraction of life before vanishing and I simply had to finish the job. I have to imagine that four coordinated Mechromancers would be able to sleepwalk through the game.
Maybe that's the point. The Borderlands series isn't so much about proving your mettle as it is about having fun and collecting loot. Gaige might make the game easier, but that also means it's more accessible. And by hiding difficulty levels inside a set of skill trees, Gearbox has made a smarter, more customizable method of variable difficulty.