For me, learning about functional programming and 'feeling my way' into avoiding excess local vars, paying careful attention to side effects. To be clear: I'm talking about functional style in imperative languages, not about making the jump to functional languages (which are still less widely used -- maybe because they're not 'necessary enough').

I'm not saying my code is particularly readable to anyone but me, but functional style is something that 'clicked' when I fell into it.