Horizontal progression isn't exactly good either unless it's implemented a certain way. Even so it can come off more as busy work to keep you away from what you're actually interested in doing, especially if said items/skills are considered mandatory for content.

"Oh. You don't have X item?"
"I don't. But I know what to do-"
"GTFO noob, we don't want your kind here."

Is very much a community killer.