I'm not saying medium is great at that - they should have implemented reddit-like discovery system, with ability to sort by top posts of a year/month/etc, transparent front page algorithm, and browsing tags you are following separately from subscriptions. But they still did a lot of things right.

A platform with already existing audience, and a convenient subscribe button can make a big difference in the number of people who follow your stuff.

Clean design and WYSIWYG editor are very valuable as well. I'm a programmer, I prefer to use org-mode and markdown for my writing, so it was surprising to me how much I enjoyed writing on medium.

Beautiful design and being able to immediately see how awesome your final article looks, is, unexpectedly, really inspiring and makes the process of writing more fun.