Manual:Extension support/1.18/ExtUpgrading

Namespaces
There are now new gendered namespace aliases. You will need to review your code especially if you construct links or have not yet upgraded to Language::getNamespaceAliases [1.16] from $wgNamespaceAliases [pre-1.16]. Specifically Language::getGenderNsText is new. This mostly affects people using NS_USER but you have no guarantees translators will only use it for such.

Blocking
The architecture for Blocks and blocking/unblocking/viewing blocks has been rewritten virtually from scratch; you'll need to review your code if you work with any  objects.