In Asheron's Call, one of the first 3D MMOs, there was a hardcore PvP server with no safe zones, no level restrictions, etc. You could (and would) be attacked at any time - in town, doing quests, etc. It lead to a really great dynamic where things that happened in game seemed really important, and guilds became very political and almost military alliances that were necessary to live a somewhat safe life and own a town, dungeon, etc.

I joined a guild that was in an alliance that held the best city in the game. Various guilds were always at war with us to try to take the town. Probably my best memory was when this group literally called The Dark Alliance, because it was the most hated guilds on the server, formed to take our town. This lead to a full-on war where we'd be online for hours every day standing around ready to fight, either waiting for them to raid us or going to raid their strongholds. It really felt like being a part of an important cause... you were out there fighting with your allies for a cause.

Probably the craziest moment was when one of our guilds, which had some of the best PvPers on the server, they switched sides on the battlefield, with their leader dramatically swearing loyalty to this guild we thought was pure evil. We immediately were attacking our former comrades just in a blind rage of betrayal. Ultimately we lost the town and the alliance fractured, and my guild ended up moving to a backwater (but strategic) town.

It really felt like being a character in a good fantasy novel. Everything just mattered and seemed so important.