Very — but with a special flavour. It literally says 'in a way you do not often meet', so when you call someone uncommonly kind, you are saying their kindness is above what you usually see. It only strengthens adjectives and other adverbs (uncommonly tall, uncommonly well), and it goes with good qualities far more often than bad ones: uncommonly beautiful, uncommonly clever, uncommonly generous. You would not usually say 'uncommonly stupid' — for that a speaker reaches for a different word. The register is formal and slightly old-fashioned, the voice of a book review or an older, well-read speaker; in ordinary conversation people say really, very, or incredibly. Its cousin remarkably is the modern everyday choice. There is a second, plainer use that just means 'rarely' ('he uncommonly speaks of his childhood'), but that one is rare itself and sounds awkward to modern ears — you can safely ignore it.