The sticking regulatory points have to do with "must have" content being combined with content distribution. WB doesn't fall into that category since the bulk of their distribution and revenue comes from the box office, home releases, and consumer products. The content that falls into those categories more than any other is news and sports. NBC has Comcast covered in both those arenas already, WB is in neither of those.
Like Universal content today, WB can be freely licensed for broadcast. The real money to be made is going to be in OTT licensing. Oddly, Comcast seems to be fostering a good relationship with Netflix for their content. So they could easily make additional money from that licensing. It's a bit of a change from how combative they were before with them. In fact, continuing with this relationship for the immediate future would work to their benefit on a regulatory front.
Channel wise, NBCUniversal has several weak areas that could be plugged from a transaction with Turner. Such as children's and international.
Report: Comcast Expects to Make Offer for Fox on Wednesday