Most of the time, it's easy to ignore programming languages that you aren't familiar with. In day to day programming life, you're not going to be running into many examples of Eiffel, or Dylan, or D or E, in the wild. So you're not going to feel very strongly about them, either pro or con.

For better or for worse (I think for the better), CoffeeScript is fully interoperable with JavaScript. The runtime characteristics and the lowest-common-denominator ES3 support are such that you can use it just about anywhere you might want to use JS. That, and a modicum of success, means that JS programmers sometimes stumble across it in unexpected places, only after popping the hood. e.g. "I thought I was just using a simple library -- Eauggh! What is that stuff down there!" This makes it very hard to ignore...

As a JavaScript programmer, you're forced to ask yourself the question, "Why haven't I given CoffeeScript a real try yet?" Not all, but a large part of the dislike comes from folks rationalizing their response to that question to themselves.

tl;dr, CoffeeScript is almost perfectly situated to inspire fear and loathing in JavaScript programmers with closed minds.