I assume it's referring to the fact that 'this' changes in a given piece of code depending on how it's called, whereas other vars are lexically scoped, so you are often forced to work around the issue by making your own lexically-scoped equivalent, aka the old "var self = this;" trope.
and this is perfectly fine. it allows you to do more if you know how to use it. and if you don't - it is really simply to understand. again - this is how js different from many (all?) languages out there, but why it is bad - i am not sure. extra line of code in some cases? I don't think this is really that bad.
I won't say it's good or it's bad, but I will say that it bites me on a pretty regular basis. You start with some code that says this.something() and you decide to move it into a callback and miss converting the 'this' references to 'self' because in the previous incarnation, it didn't matter, but now you're calling out to code that helpfully fixes up the 'this' reference when calling your callback. I literally fixed one of these today.
Are you using strict mode? It doesn't fall back to the global object as `this` (except for setTimeout/setInterval, which are specified that way) - since I've started using it, I find out pretty quickly if I was missing a `.bind(this)` when passing a callback function.
