What is wrong with oceans and mountains:
1) In mountains, the grass doesn't regrow because of bugs. Apparently underground is affected by this as well, including under oceans.
2) Because since 0.31.19, they are mostly covered in a thin layer of soil, it is not possible to engrave a mountain in the ways it used to be.
3) Because it is possible for a few fisherdwarves to fish an ocean clear of fish in a short period of time. I know this bug is fixed as of the next release, but it is still annoying.
4) The waves in oceans cause massive FPS loss.
5) Good and Evil mountains don't have dwarves, or in fact any sites of any kind but caves. In fact the normal mountains that have dwarves don't have any evidence of their existence.
6) There are no civilisations or other entities in Mountains but dwarves, and none in oceans, and none can utilise mountains for fun purposes like sending over raiding parties or doing things other than obtaining resources.
7) When you set a large number of good and evil tiles, they tend to congregate on mountains and oceans, because those are large regions. Really the only way to do this properly is to set a large number of small and medium sized good and evil region tiles, without many large good and evil region tiles, and even then it is not guaranteed.
Because of this I try to avoid settling in mountains or on oceans.
I agree that both oceans and mountains are rather boring, and the FPS loss on oceans makes it even worse. In order to make them interesting, we'd need to have some ability to build boats or ships in the game, and have a larger variety of events that can happen around them. The current undead sea creatures do not count, because most of the time they just stay out in the ocean and at most will scare your dwarves off from building above the ocean.