The trick is that you have passive cooling (a heatsink) in what could be a completely enclosed space (like a glass shade/globe on a flush-mount fixture). The heatsink can't be too big to occupy a similar volume to a 'normal' A19 bulb, or it won't be able to fit in lots of existing fixtures. I suppose we might see some bulbs try active cooling (with a fan on that heatsink), but people are already very aware of hums and buzzes from their fixtures, so it'd have to a quiet one.