Yes, BDSM aspect aside, "50 Shades" is basically Mills and Boon. (In fairness, I haven't read it, but I have read a very detailed blog about it which made me decide not to.) ;)

Possibly a m/m romance (not necessarily particularly well written but with a classic romance storyline) could be a breakthrough success if the m/m aspect was perceived in the same way as the BDSM one - as the exotic, forbidden ingredient that makes it different from all the other romances on the shelves. Not ideal, but if it gets readers to expand their horizons and possibly read other, better stories, it could work out well. :)