Not had much time to play so I'm still only at Act 6, so far the changes have felt pretty decent. Act 1 was certainly a bit tougher but beyond that it's feels like it's mainly the bosses and unique enemies that are more of a threat. The main thing I have felt is mana cost increase, that's been consistent so far. Obviously early days yet and maps are a different story so things could go south pretty quickly.
As for the league mechanic, I'm actually enjoying it more than I thought I would, it feels pretty rewarding so far, especially haggling with the currency trader. Only small niggles so far are that I wish you could detonate from anywhere rather than running back, plus it feels a little bit janky on occasion in the indoor areas where you have to land on the patches of ground.
I've gone Hexblast ignite for my starter, frustratingly though Hexblast is bugged at the moment as the skill animation/particle effects and sound are missing. Feels very wonky as there's no feedback to you actually landing a hit. The Skin of the loyal chest I should be targeting is also bugged as none of them are rolling with blue sockets anymore, It's probably going to make them very expensive when it gets fixed.