IdentityServer is a really good product but:
- Only authenticates against SQL not AD
- Doesn't have SAML support - useful for third party integration e.g. SalesForce
- No rich claims rules language
which ADFS does for you.
@leastprivilege answered the first - the rest are definitely not trivial to add.
Don't have much experience with Office 365 but it sits on top of Azure Active Directory which can be hooked up with IdentityServer - refer Federating IdentityServer with Windows Azure Active Directory.