Shudder. Flashbacks to having to write optimiser hints in Oracle (and the resulting fun times when you'd upgrade the database, something would change, and your hints would make a query slower).
Most devs have problems writing decent queries (in some situations), now you want to introduce writing the query execution plan into the mix ?
The worst thing that could possibly happen is that you give it bad advice leading to slow queries, and then the obvious first step to fixing that is to drop the manual advice and see whether the automatic planner handles it better.
It baffles me that PostgreSQL, which is so deeply customisable in almost every other way, resisted this form of customisability for so long. This is great news.