I think DC would be wise to market especially to DC fans with the triple-punch of classic TV/movie/animated content, new TV content, and especially the comic catalogue.
I know a lot of DC fans that aren't subscribers and are surprised when I describe the comics library to them.
It feels more like they're marketing a streaming service with not all that much new content and I'm not sure that's their best argument.
On classic/old TV/movie content: It's a shame they launched at a time they weren't clear to include modern DC movies/CW stuff (even if it's pretty bland) because of pre-existing contracts with other streaming services/channels/etc. If they were able to sell DCU as a clearinghouse for ALL DC content, I think they'd improve their numbers by magnitudes. (It might have been worth waiting as a business model but DC doesn't tend to wait about anything but Doomsday Clock. It's all about chasing the next new idea without a lot of forethought. See: The New 52.)
On new TV/animated content: Even though we've gotten about one new episode a week since the launch of DCU, I feel like an episode a week has a feeling of slightness. I think the service might be more attractive if it follows the Netflix/Amazon/Hulu model of dropping a season at a time for binging purposes. I can think of one good reason they aren't doing this and that's that people might be more likely to do free trials or one-month subscriptions and then cancel. I think that might be shortsighted. If they could capture more free trial/one-month subscribers they might well win a lot of converts with their comics library.
On the comics library: I can't say enough about how great it is. I used to trade wait some books and pay about $15 to get ONE collected arc. I'll still always buy a lot of comics the day they hit the stands but there are SO many comics I'm willing to wait a year for, knowing they'll ALL be available for around $8 a month before too terribly long.
Doom Patrol is tied for my favorite live-action DC anything of all time (nothing can ever surpass Batman 66 for me so a tie is a really high compliment on my part) but, to me, it's the comics that are an unbelievable value.
Every time I see a thread here about great stories I missed or am reminded of great stories I haven't read in a long time, I go straight to DCU and read them. I don't have to dig out my floppies. I don't have to wait for a comixology sale. Almost every great DC story I'd ever want to read is only a search away on DCU. I love it as a streaming service of great, new content but I REALLY love it as a DC comics library.