Here is a list of features currently available in Libravatar and how it differs from Gravatar.

Libravatar Gravatar
JPEG and PNG support Yes Yes
GIF support Yes No
Upload size limit 10 MB None
Image size (in pixels) 1 to 512 (configurable) 2048
HTTPS support Yes Yes
IPv6 support Yes
Supported email hashes MD5 and SHA256 MD5 only
OpenID URL support Yes
Redirection to Gravatar Yes Not needed :)
Customizable default image Yes Yes
MPAA-style image ratings G-rated only Yes
Text-based profile information Yes
Federation via DNS SRV records Yes
Persona authentication Yes
LDAP authentication Yes
OpenID authentication Yes
Automatic photo importers Yes
Translated user interface Yes Yes
Account export Yes
Account deletion Yes No
Libraries/plugins available Some Lots
Free and Open Source software Yes No
Distributed service Community mirrors Commercial CDN
Corporate owner Automattic

Is something missing from this list? Let us know by filing a bug, or contributing a new feature yourself!