Marriage not dating is one of the classic korean drama with romantic theme. It is infused with the whole 10' trends, but, part from that (though nostalgic packed), the story is okay and entertaining. Sure, the clisse rich mother doesn't want the bride is all over this drama, but it has some fresh notes. Marriage not dating is actually a daring drama, with many themes that usually K dramas don't lay a finger on. Also, the performances were good from all the cast.
Now, about the things I didn't like: 1) the chemistry of the leading couple wasn't very intense as it should have been. 2) the girl was a bit off for the her role. Not the actress, who was excellent, but the character was a bit odd and didn't fit the story. She drank a lot ( a few scenes would have been sufficient to understand her, and not make her look like an alchoholic), she jumped from one guy to another (she claimed to be in love with the first guy, and, in episode 2 she wasn't anymore....), she was behaving like a child at times and she was kind of annoying. 3) at some point, the drama had a confusing point.
So, 5 out of 10.