From a Barbarian's perspective, inferno was absolutely perfect in terms of difficulty (especially act 2).
Act 1 was extremely manageable. Act 2 was a big gear/play style check; it was difficult enough that at times, it seemed impossible. However, with the right gear, act 2 will eventually become as easy as act 1. Act 2 was also a big wake up call that made you re-examine your gear/skill choices.
Act 3 was a much smaller gear check than 2, and Act 4 is just a loot pinata as Act 3 and 4 are almost identical in difficulty.