I wouldn't paint all hints with the same brush - it's not like the very fact of having a hint exposes query planner internals for arbitrary usage. Some hints may be more useful than others, and some may be less complicated to maintain - why not try to investigate if there's an intersection of these two sets that would be a valuable addition to Postgres?
Maybe. The traditional query hinting implementations like MySQL's are all about exposing planner internals, but maybe if someone proposed a form of query hints which is less intrusive then there might be fruitful discussion about it. I think a huge issue is that as soon as someone mentions query hints they directly think about MySQL's and similar solutions.
