I joined DA in 2007 after years of having just browsed and never had much luck with it. Yeah, the OCTs and SDL were really fun and helped me build on my skills a bit, but most of the artists I grew to like left DA to either start webcomics or post on Tumblr. I myself found that it was rather unfriendly towards Traditional media--I had quite a few comments in my small comment pool that insisted my art would look better if I made it in PS. Wow, thanks.
In addition to all that's been mentioned so far, the one surprising thing is that DA's UX has stayed relatively untouched since it was first conceived. And it is clunky as hell. The process to upload one piece to your gallery is five million steps. And you still can't upload gifs without a preview image, which is such a hassle (I save a lot of my B+W stuff as gifs for the web). With all the senseless and extraneous updates made over the years, why are the interfaces still the same? There must be about 5 different ways DA could update their info architecture, basic UX and UI, etc....to create better ease of usability. This is comparison to Tumblr, where I can upload any kind of image (or video, or sound, or whatever) in two clicks or less, easy as pie.
When they did the logo redesign, they just looked desperate. I think if DA wants to stay in the game, they need to seriously evolve the look, feel, and usability of their platform. A site that hasn't changed their basic layout in over a decade? That is unfathomable to me.