Part of the allure of a horizontal endgame is that you can play alts freely. You get a whole stable of them, and you play what you want to play tonight (or what your guild needs) rather than being locked into the one character that has raided enough dungeons to be viable in the current endgame. I made 11 level-capped characters in City of Heroes and was only frustrated by the introduction of an endgame system that added character-based advancement.
Guild Wars 2 spent the first year gradually devaluing alts by adding character-specific advancement. The second year has started converting that to account-based progress, adding value to alts.
Most of the Living Story content is character-based, for example your abilities in the Super Adventure Box or which floors you can travel to in the Tower of Nightmares. Every tier of gear-based advancement is character-based advancement, so the ongoing rollout of ascended-rarity items has pushed against alts, although more slowly than the trivialization of acquiring exotic-rarity items has made alts more convenient. Oh, and Belcher’s Bluff signature moves.
There were two big additions of character-specific advancement that are now or will soon be account-based: Fractal level and WvW experience. The single pool of WvW experience (with different point investments across different characters) seems like a great solution there.
I was surprised to see Fractals going account-based just because there was no solution that would not upset some players. Add up the Fractal advancement across all characters? “Easy mode” was just rewarded for having many characters do the low-agony levels. Just go with the highest level? That throws away a lot of character-specific advancement for people who ran Fractals on multiple characters. ArenaNet went with the more extreme plan of not only “highest level” but also “and we’re capping that at 30.”
If the plan is to convert to more account-based advancement, the next one on my list is unlocking explorable mode on dungeons. I have completed story mode of every dungeon on at least one character, but I keep a spreadsheet listing which characters can start which dungeons.