I'm afraid that I'll become too complacent as a developer, and that I'd become too entrenched in a certain methodology, paradigm or language and that I'd be unable to learn new things.

Coming from a OOP background, functional programming sounds interesting, but scary at the same time.

I tend to do things instinctively, which can be useful, but it also means that I don't learn things by the concepts - which tends to be a problem when you're trying to learn more.