Once again, those Wikipedia articles are highly recommended reading.
The main example I pointed out is American politics. There is quite a bit of diversity of opinion and that makes it far more difficult to reach a consensus about a particular issue than you'll find in e.g., European and Asian countries, which have far more homogeneous populations (and by extension, far more homogeneous political/social opinion). As a result, the speed with which progressive policies can be implemented in America ...lags... our European brethren, to put it lightly.
I pointed out that you were talking like a Nazi because, in this instance, you were talking like a Nazi. Do I think you secretly harbor a desire to wipe out an ethnicity via genocide? Nope. Do I think that criticizing ethnic diversity as standing in the way of progress sounds suspiciously fascist? Why yes, yes I do!
Identity politics has no doubt hamstrung the American left, but that has had little to do with your cited example of the failure to implement a nationalized healthcare system. Diversity increases viewpoints, more viewpoints invite stronger debate. Does less debate lead to better outcomes, or faster outcomes? Which is preferable?
There have been books written on why nationalized heathcare is hard to achieve in the US. It's a very complex issue that is inherently entangled with both the mechanics of the US political system and the US media. Blaming it on a "diversity of viewpoints" does a disservice to the issue.