I look every once in a while for other forums similar to Piano World/Pianist Corner that exist for other instruments but have always been disappointed with what I've found. Is there, for example, an "Oboe World" or an "Art Song World"?

You mean you don't want learn to play the piano? 20 minutes a day and 600 dollars for a digital keyboard with weighted keys that you can play/practice 24 hours a day with a headset. Anyone who can play a double reed instument like an oboe can easily learn to play and love the piano and add the piano to your list of instruments that you can play. Think about it?

Where did Arghhh say he/she didn't want to play the piano?

Maestronet is a string forum I used to visit. Similar to PW in some ways. I posted there a bit at one stage and found some helpful advice, but, like here, you have to hang around for a while to work out whose advice is worth listening to.

For classical guitar for example Delcamp is really good, with many resources too, although I am mostly a lurker. For the flute there are quite a few, but I haven't found one as active as PW or Delcamp yet.