dislikes:
Shitty, immature writing styles where the author just blurts out what happens. Phrasing/sentence structure never adds to the mood of the situation. Even if the idea is original, really underdeveloped writing just sticks out like a sore thumb the entire way through. If the author doesn't get better after 50-100 chapters I usually drop it and never look back.
Underdeveloped characters and recurring side characters that don't really seem to serve a purpose. A diverse/well developed cast is fantastic if you can manage it, but the majority of the time you get a bunch of filler characters that aren't fleshed out beyond grimdark angsty mean dude or moe moe little girl. Especially unrealistic if the author makes them revolve around some cardboard personality MC instead of having their own wants/desires/goals; that shit's so silly it ends up breaking suspension of disbelief. LMS, for example, is super guilty of this. IMO it'd be better to just have a few really well developed characters/villains that you can fully empathize with or hate with all your heart if you're not very, very confident in your writing ability.
Stupid/immature MC whose awful decisions get used as a filler plot device to artificially insert conflict rather than a well thought-out confrontation. It's fully OK to have that kind of behavior if a significant aspect of the story is growth of the character (either through events or by having the timeline of the story span from the MC's childhood till adulthood), but most of the time it's not that way. You usually end up with a really frustrating retarded MC (later chapters of CD, looking at you), or the story reads like a 14 year old anime nerd's wet dream (it's almost at the point where the harem tag is a warning sign that the LN is gonna be awful)
Examples of really well done web fiction (imo) would be ones like ISSTH, way of choices, worm (wildbow). I'm sure there are some more on the front page here with similar writing quality, but I dropped a lot of them early due to personal taste so I can't comment on them.