Keep in mind that Justice League #12 was
the conclusion of Johns first year of stories on the series so Diana and Clark were basically actually secondary to the actual main plot of the books which was the League dealing with Graves. That was one of the things that I liked about the issue.
"Fan-service"? I don't know that many fans who are actually enthralled with this relationship Trekker, so I'm not sure what you meant by that comment. The relationship feels odd, because it's been forced upon us. This is basically DC's version of One More Day. It would have felt more natural had they not gone and "erased" Lois and Clark and this was Clark moving on, as Lois has done. But nope they basically have just tossed this in our faces. Like so much of the New 52. This is from someone who actually liked the issue and thinks this relationship has potential. I wanted to hate the issue to be honest, but I found myself liking it. I just dislike the way DC has introduced this particular plot.