Although SE in general considers tags to be equal, we have some tags (like achievements) that are usually only valid when paired with another tag for a particular game or series.
However, the code for suggesting related links doesn't take this into account. For example, say I am viewing a question on a particular Diablo 3 achievement (tagged diablo-3 and achievements). The 'Related' widget may be suggesting achievements questions from other games, such as Mass Effect or Skyrim, when it should prioritise other Diablo 3 achievement questions instead.
This may be kind of a unique feature request based on Arqade's domain, but I think it'd work better if 'Related' sidebar links gave more importance to having other tags in common.
We should also consider not allowing questions to be tagged with only achievements (no idea if there are such posts or how to search for them)