When the PCs are all (or overwhelmingly) somehow starkly unusual--e.g., the evil campaign--it seems like it can't help but influence the direction of the campaign, and in potentially unpredictable ways.

Whispers among my group suggest we might do an all-undead-PCs campaign, and my inclination is to mandate each PC to be a different type of undead, so as to make the crew a bit more motley and hopefully keep the mood lighter than it could otherwise get. ("Can a vampire, an awakened skeleton, and a quth-maren find a way to get along?")

I certainly know about the mechanical/power issues at stake--e.g., incorporealness breaking encounters, fatigue immunity. I'm more interested to hear about STORY issues--things that happened with the campaign's plot arc(s) that the DM and/or players didn't expect. I particularly want to hear about how these stories ended.