Well, there are drawing styles that turn me off at the start or when I see people copying a certain style way too much so that the comic feels like a copy-paste of the 10 stories that I read before. There was a time I scrolled through a front page of a comic hosting site, and it felt like I was browsing a gallery of just one artist as all the covers looked very alike. It's not a bad thing, but do you know the feeling of eating the same thing over and over again until you get sick of it? That's what I'm feeling about it. I've just read way too many comics to digest any more of the same style XD
When the boys in a BL comic look "famished". Like, you can see every bone in their body popping out, making them look very stiff and honestly, terrifying. I know a slim waist and defined collarbones/shoulders are considered sexy, but don't overdo it!
The cliche of the main character in a BL is SUPER-NO-HOMO™ that is thrown into an awkward gay situation, and then the next 10 chapters are focused on him doubting his sexuality OR repeating over and over again that he's not gay, even if he admits a page earlier that he enjoyed kissing the other guy or something. It's one thing when a character is confused and does some self-discovery and another when his world is suddenly turned upside down because he held hands with a guy for 5 seconds.
It was funny the first 20 times. Now it's just meh. I'll let it slip if the character design is a direct hit into my aesthetics, but otherwise I'd rather look for something else.
(Naturally there are exceptions to these, and I will enjoy some comics even if they have things that normally throw me off.)