How so? For my own curiosity and future Economics major wannabes 
I'll probably end up doing a full length review in the subject review thread, but basically there's an extreme focus on mathematical notation and proofs. I understand this is a mathematical-based subject, but come on if I wanted to study real analysis I'd have done a math major. They try to remove the advantage that students who've done real analysis have by not being too harsh on notation in assignments, but some of the lectures are just ridiculously hard to decipher. It gets to the point where the lecturer is basically spending 20 minutes to derive a proof and most of us feel overwhelmed. I've done Linear Algebra and Engineering Math, so I'm no slouch, but I find some of the proofs here ridiculously hard to follow.
Also, it goes the complete other way too. There are times when the math is conceptually pretty straightforward, but there are LINES AND LINES of differentiation and optimization and rearranging. Most of the difficulty in these sections is remembering not to forget a negative sign or multiplying numbers correctly, it doesn't feel like "economics" at all.
Basically the subject is "lets look at the mathematical intricacies of this model under this super limited set of assumptions". It just feels like we're doing real analysis and rearranging algebra for the sake of it. The scope of the examples are so limited and the models are so abstract and far from reality that they border on useless.
That said if you enjoyed proofs and real analysis then you'll probably like it, but it's not for me (and the vast majority of my cohort, from what I've gathered).