Avoid the “Jack of all trades, Master of none.”-mantra, as this not only defocuses & complicates your execution, but it most definitely decreases the quality of your work / products / services.
— Do One Thing by Adii Pienaar
It's something that I have to rectify. My development career has moved from one programming language to another without me taking the time to truly specialise in one specific language. I think I've just found my theme for next year.