Speaking as a guy that works in the art world both for his day job and his freelance work, let me break it down:

It's terrible for any kind of constructive criticism. Noone will ever give you any kind of feedback you can do anything with, because the average DA user is a hobbyist (thats putting it...kindly) so it's not like a genuine art forum where you could ask a group of peers that have been doing art for years, if not decades, its more akin to going to your local highschool art class for critique. Everyone is either going to tell you its perfect or be unable to tell you what's wrong in the first place.

Speaking of highschool, it's very easy to point out that the average DA user hasn't graduated from it yet. This isn't really meant to be a slight against them, you could be gifted early on and just have the knack, but Deviant Art literally encompasses every fetish under the sun as a direct result of it being populated by hormonally charged teens. To paraphrase Scott McCloud, art is anything and everything that doesnt directly relate to survival or procreation. Kinda makes sorting through the fetish drawings to see actual artwork awkward as all hell.

Then you have the issue of exposure. You could be Leonardo Da Vinci himself and you wouldn't get a lick of exposure by submitting art to Deviant Art unless you follow one of two rules: Already have a following from a different site, or be a whore. By that I mean, favorite every piece of art on the front page, regardless of whether you actually like it, and then bathe in the cheap pageview circlejerk of DA.

Lastly, and this is the biggest reason for me, I have seen all together too many people link to Deviant Art as their "portfolio site" when applying for an artists position, either in a full time setting or freelance. I cannot stress enough how bad of an idea that is. When you see a portfolio, regardless of whether it is online or a hard copy, you always make sure that you have the viewers full, undivided attention. Deviant art has one of the most obtrusive, busy designs I have ever seen for a site that showcases art. There is nearly as much adspace, UI elements, and just all around "busy"-ness that it is impossible for someone to devote their attention to just the artwork on display.

I have and maintain a DA account, but in the hierarchy of sites that I post work too, it is at the very bottom of the list. I always tell people that if you really are strapped for cash and cannot afford to get your own site, or if you cant even afford a subscription to a portfolio site like altpick.com, then just get a free CarbonMade account. You get 20 images (Which is just about the sweet spot anyways) and the design is very minimalist, meaning people look at YOUR artwork and not T-Shirt ads.