There are plenty of minor things that I find annoying, but I think his biggest true toxic trait is a lack of confidence. It means he overthinks EVERYTHING, especially at work. I don't know how many basic emails he's had me look at, usually after spending an inordinate amount of time working on them before showing them to me. I always think they are going to be something lengthy or major but nope - they are like 3 sentence emails that I never would have given a second thought to. It wastes so much of his time.
Along with that I guess it's feeling like he has to be working all the time. He doesn't ACTUALLY work all the time but he'll spend the whole weekend talking about how he is going to start work in an hour or can't make plans for Sunday because he has to work. He does have a heavier workload than me but I think a lot of it is self imposed.
He wears socks like a normal person when he is out and about in shoes. But when he gets home and has his slippers or house crocks (lol) on, he wears a sock on only one foot. He claims it keeps him from getting too hot, lol