The comparison to Helvetica and Garamond misses the point of Comic Sans, which is to be a casual, friendly and playful (as opposed to professional, austere, serious, business-like) font that looks more handwritten than typographed (like the handwritten captions in pulp comics of yore).
Even the kerning and height issues they point can be explained for that. Perfect kerning and proportions would ruin the illusion of it being handwritten and childlike.
As to why all the hate, it's easy: middle class people with any "artistic" or "creative" inspirations (and I use the term loosely) will jump at the first chance to feel more sophisticated than the great unwashed masses. And designers are the very epitome of that crowd [1].
And most will just jump on any bandwagon related to such a cause (notice the hundreds of anti-Comic Sans blog posts, articles, even t-shirts), without thinking the issue thoroughly and trying to understand neither the original intent nor how and why people actually use the font in practice (which is even more important).
The real question to ask is not why designers and hipsters "hate" it, but why the huge "unwanted masses" adopted it with such eagerness. But that would imply that the average designer also has something to learn from the people, instead of preaching his dictums (which most of the time are not tied to any science or real world observations at all) of "what should work" and what "they should like" to them.
[1] The same people that, in their web incarnation, blathered on and on about "semantic markup", missing the point of both "semantic" -- a word that they repeated as parrots from one another after they learned it from a few misguided trend-setters -- and "markup", which is not supposed to be where you convey semantics). This, now mostly subdued trend, kept the web 5 to 10 years behind -- thank god for SPAs, flex and grid layouts and the like.