Aesthetic turn-off:
Blurry backgrounds. It's a popular way of differentiating background from not-background, but I hate it. I guess the idea is that the stuff might be less focused if it's further away, but in practice, with properly corrected vision, I don't find this to be the case - in fact, if you told me that you thought the blur effect was realistic, I'd implore you to go get your vision tested. Because, honestly, that's kind of what it looks like - a nearsighted person looking at something far away without their glasses/contacts (this can be observed really well if you have bad, but corrected vision, because then you can put on/take off your glasses for a quick comparison). Of course, technically you can't focus on something close and something far at the same time, but it's not really noticeable IRL, because if you shift your attention from something far away, your focus will adjust, and the fact that the stuff you're not focusing on isn't super clear isn't all that noticeable, because you're not focusing on it. If you try to compensate for this via a graphical effect, the problem is that it doesn't change if the user shifts their attention from the play area to the background, which means the background just looks like garbage all the time.
My suggestion: Prefer differentiating background from not-background via color contrast as opposed to blurring.
Bonus turn-off points: the menu background in NL has a blur effect. On one hand, there's not much going on in that background, so it at least doesn't draw attention to itself, but once I notice it... cannot... unsee...