Windmill is not broken in the slightest. Not only would nerfing or removing it basically make melee specialization virtually useless (without WM, literally every specialization does what it does better), but using Windmill as a main means of attack takes quite a bit of skill in terms of positioning, pet control, not to mention that to maximize DPS, you need to detract from your survivability by not using a shield (and passive shield defenses actually ARE broken).
In any case, OP's premise is laughable; yes there are a lot of missions/challenges that you don't need a high level of teamwork and strategy to accomplish and can just power through. But as Cryosite said, if stuff didn't become easier, what would be the point of leveling up? But there are lots of things that DO require teamwork and strategy (looking at you, Fortinbras' Expedition), even for high-level players. If you want to re-experience struggling to clear Ciar Basic, roll a new char.
I'm not advocating for a removal of windmill, or a crippling nerf. Something as small as removing its ability to crit would do the trick since then it wouldn't double as best defensive and best offensive skill. I'm also well aware that using only windmill takes skill and hinted at it, though I didn't explicitly state it I suppose. In those situations, removing the crits would only make it take longer but would not make it impossible.
Without windmill, melee still has a lot of perks. For the most part, I completely forego milling and still am a demanding presence on the battlefield. This is mainly because, with a battlehammer or knuckles, windmill is far less useful than dual-wield.
I don't think the OP is complaining that things are easier, but rather that we're being force-fed a faux difficulty rather than a true challenge. G1 is worth mentioning because back then a confrontation was decided by a players skill, for the most part. Now, we're using the same tactics...but we have to repeat them several times to actually kill a monster. The pacing of the game suffers as a result and the fun factor takes a nosedive. The only 'challenges' are things that can't be dealt with entirely through player skill such as massive multi-aggro.
To be fair, I think the theatre missions are a large step in the right direction. The monsters don't have nauseating hp/prot values, but it's still challenging enough to warrant not coasting through on auto-pilot. Missions are short, sweet and to the point...and you don't have to spam beginner levels to get to the stuff you actually want to run.