More enterable buildings, please!

[Catwho]

There have been a few actually locked doors that were opened up at some point. The crypt in Uldah was locked until they added the second chunk of alchemist quests, and suddenly you can go into it at any point, no gatekeeper required.

That said, the majority of the gatekeeper NPCs in front of doors are because they didn't want to have a bazillion player characters when that content was added, and by allowing it as a separated instance instead, you can maintain the privacy of the area. It's bad enough to have 100 players crowded around an NPC when it's outside. It can become unmanageable in a small room.

OP's initial point is right though: Part of the fun of MMOs is exploration and little easter eggs, and there are too many locked doors. One example that comes to mind is the Pelupelu society - all those cute little cottages for the visitors, but there is not a single one we can enter! Not as part of a quest, not as part of a cutscene. I tried to get into them all. They're pretty set decoration, but they don't even feel like they are being used or lived in because of it.

There are lots of places in the game that you can enter, and if the NPC sees you, they shout "GET OUT!" That's awesome! Not every door should be locked with a "NO ENTRY" sign if some people do need to be there for legit purposes.