Horse Girls: Obviously, from the cover art and title.
Fan Service: Not really. Through 4 episodes this show has taken the high road and chosen to avoid too many opportunities to pander to that demographic. So for those hoping to see Horse Girls in often compromising situations, I feel this show will disappoint you. Episode 1 is your one fleeting glimpse of anything approaching it.
Idols: Unfortunately, and it feels tacked on, as an afterthought. Fortunately they don't focus too much on it, but I wish they would have used the time they do devote to it to develop the characters or plot further instead. Without the Idol bit, I feel this show could have pushed into an 8 by seasons end.
Comedy: I am enjoying how they are treating horse girls as a concept. It's made for some mildly humorous events between certain characters and situations. There is more comedy hidden in the antics of certain horse girls, but only those who recognize who they are (see below) will probably get it. They even managed to make the Idol portion of the show somewhat humorous. None of which I think will make you laugh in any audible sense, but you might smile or get a little warm feeling in that part of your brain that tells you something was funny in a cute way.
Drama: Maaaaaybe we might see some later. See below. For now the show seems pretty laid back and campy.
Sports: This seems to be the bread and butter of the show, so if you like sports anime you'll probably find plenty to like in this show. So far the races have been entertaining to me, and realizing their historical significance has increased my enjoyment of them and the plot as a whole. The girls seem to be friendly rivals, but there seems to be more of a competitive rivalry developing where the horse trainers are concerned.
Speaking of historical significance, all the girls in the show are named after real horses. If you don't want to spoil yourself, then don't bother looking them up, but if you're like me (who isn't too bothered by spoilers) and wanted to know if they were real names and more about them, you might wind up getting more enjoyment out of the show by educating yourself about the horses and their racing heritage, and that enjoyment includes some of the antics some of the girls have displayed so far.
So that's it in a nutshell. I've been enjoying the show. The girls are cute (cute character designs in general), the story is interesting to me. The art and sound seems good to me, but in fairness I'm not the kind of guy who runs around critiquing things that closely. So far this has been a pretty good Cute Girls style show that happens to be more of a sports anime...that happens to have Idols. I'm giving it a high 6, so we'll round it up to 7/10.
