I had never considered that a druid wouldn't
get along with a conjurer. He might have been a member of
Battle, but I didn't notice any title after his name.

All members of Battle do not have titles attached to their name. Since we are talking about a druid, a communer, it is likely that he was an Outlander and thus didn't get along with conjurers. Of classes, outlanders don't get along with conjurers, anti-paladins, necromancers and paladins. So, the druid was likely to be an Outlander, which is not uncommon when druids are concerned.

In any
>case it got me thinking about natural enemies among the
>classes that might not be as obvious as paladin vs.
>anti-paladin but still exist.

This druid hates conjurer thing was likely druid being Outlander and thus hating conjurer, not a class vs class natural enemies setup. Paladins vs A-P's and necromancers is more or less fixed animosity, as paladins are goodies who fight evil and A-P's and necro's are evil.

I can't think of other certain hate/hate relations based on class only.