This thought keeps pushing its way to the forefront just about every time I do dailies, but there is a ton of under-utilized content in the game:

• Delves: once you've snagged the skyshard and beaten the boss (once), there is almost no reason to go back to a delve. Bolgrul has 15 dailies for alliance zones, so there are ~70 delves in the alliance zones which never get touched again aside from one or two Dragonguard dailies (Corpse Garden is also on Bolgrul's list), plus a couple for the Northern Elsweyr Defense Force after that's unlocked. The six in Coldharbour aren't used at all, but some in Craglorn have a repeatable.
• World Bosses: expansions have dailies for these guys, but the ~100 in base-game zones have nothing other than the achievement for beating them and an occasional event reward.
• Public dungeons: none other than the 15 in alliance zones which are the target of Mages Guild dailies, so 18 in expansions and I'm not sure how to count the Atoll of Immolation in Blackwood/Deadlands, which is kind of a hybrid of public dungeon and world event (and has no daily as either)

I'm leaving Cyrodiil's 18 delves off the list because beating any of the delve bosses gives a timed AP buff, so there's almost always a good reason to dive into one of those.

Seems like kind of a waste of good content to have close to 200 locations which are mostly the object of a one-off side quest.