I lose the btrfs functionality of it repairing bad blocks. I'm pretty sure md doesn't notice the error if it doesn't come from the backing device in the form of an IO error.. So if btrfs gets a bad block from md, there's not much to be done, except to make a guess which of the mirrors has the copy and manually resync (if btrfs manages multiple devices, it can just do that automatically). I don't know how that would be even done. But at least I would know which file has the error and I can restore it from the backups. Metadata is duplicated so that shouldn't be an issue.
I also lose the ability to change the data layout at ease; in btrfs going from raid10 to raid1 to raid0 or adding new devices is just a matter of running btrfs balance (and btrfs device add). Md doesn't even support adjusting the number of devices in raid10 array, nor conversions from raid1 to raid10 (that can be done manually, but you don't have redundancy during the process).
In future I may lose the forthcoming ability to set redundancy level per subvolume or possibly even per directory/file.