Interesting, I'm the other way around, I've defaulted to classes and then found that Ids would be more appropriate in many cases.

My simple rule for id vs class (as a non-expert) is if you say 'this is the x' e.g. 'this is the footer/header/navigation-panel' then it's an Id, if you would say 'this is a y' e.g. 'this the a product/list-item/image' then it's a class.