There's three big things for me.

The first is the obvious stuff with text. If it's too small, low contrast, or the line height is too close I'm going to have trouble reading it. My eyesight is garbage so anything below about 12px is legit hard to read. I can deal with 11. You go to 10 and I'm going to start getting annoyed. Anything smaller than that and I can't read it at all. Line height should be ~120% or more. Whitespace is good guys.

The second is what I call careless responsive design. I'm glad that people are considering other monitors more, but the percentage width forums that I see are basically unusable on my screen. My monitor is basically a small TV, so it's super big and super wide. This means that if the forum is like 90% of my screen width I can sometimes fit entire paragraphs on one line, even if the font is a reasonable side. It's actually REALLY REALLY hard to read things if there's more than ~90 characters on one line. It's okay, people with big monitors are used to a bit of extra whitespace to keep things functional, so don't be afraid to abuse that max-width property.

The third and probably the most egregious with current trends, is animated anything. Doesn't matter if it's a gif image in the header or by the cbox advertising an event, an animated avatar, or even that stupid falling snow effect, if there's something moving on the page when I'm trying to read it's going to drive me absolutely insane. If it's bad enough sometimes it can even make me outright motion sick. That's particularly common in gifs with short loops that don't loop seamlessly. As much as I'd like to stop using my adblock on forums to support the hosts, I can't because too many people use gifs and I need to get them off my screen and this is the only way I really have to do it. Note that this only applies to animations I can't control. If I hover over something and it animates it's fine because I'm making that happen.