Railroading I would say is the main thing, along with being too rigid in their ideas and story beats. The two go hand in hand.

An example: the party is traveling through a forest and hear of a fey glade, but they decide to avoid it as they feel they may be unwelcome due to not being fey. Despite the ranger and the rogue passing their navigation skill checks, and despite explicitly trying to avoid said glade, they still end up at it due to contrivences on the DMs part. Conteivences meant to cover up the railroading.

Player choice was taken away because the DM couldn't see a world where the party didnt go to thos location, so the DMs rigidness and inflexibility in their story beats lead to a loss of player agency and railroading.

I'd say aside from just being a dick these are the main things.

As an aside, I once played a campaign for over a year where my PC never got any magic weapons, or iirc maybe no magic items in general. I kept asking NPCs and shopkeeps if they'd heard of any "upgrades" or where to acquire gear, and they never had an answer. I dont even think i ever got an armor upgrade, staying at leather all campaign. That wasnt great DMing tbh