What's the best way to put the data I have on a Leaflet map?
Check out this awesome cheatsheet.
Why is there still no feature X in Leaflet?
First of all, did you check out the Leaflet plugins page? It lists about a hundred plugins doing all kinds of crazy stuff, and there's a high possibility that it has what you're looking for.
Generally, we do our best to keep the Leaflet core small, lightweight and simple, focusing on quality instead of quantity, and leaving all the rest to plugin authors.
Check out this video of a talk by the Leaflet creator for more background on the story and philosophy behind Leaflet. Another essential read is Advocating Simplicity in Open Source by the same guy.