The uncanny valley effect is the first thing that come to my mind, others are the distortions of the body/faces ( that you can't always correct, this is especially evident when you use the old Genesis models).
These are the most important, followed by the overuse of standards assets ( mainly hairs but other physical assets are in the same league), and in general the fact that if you don't know how to use DAZ really well, renderings are less pleasant to the eye ( especially due to the fact that DAZ is way more demanding in terms of hardware when making the renders, so if you don't have a very good machine you can't use it to it's full extent).
But as I wrote it's a personal preference mostly, there are many other users that don't like HS/HS2 for the same reasons so take my "critique" if want to call it like that with a grain of salt.
If the story is good enough and the game is engaging I don't care which graphic software is used to make it I will play it regardless.