Well, there is a game design sub-forum here, and if you're still getting technically-based feedback that you don't want, you could always state up-front that you're explicitly not seeking technical feedback centered around feasibility or lack thereof. It can be somewhat difficult to hold game design discussions that are divorced of technical concerns, so the assumption tends to be that designs must be grounded in technical feasibility -- you'll have to explicitly disclaim that if you do not want or need it to be a concern for your purposes.

Frankly, there aren't many game development forums at all outside GD.net, and probably none of them are any better for non-technical design discussions of video games. If your design mechanics are more traditional (of the type that could be expressed as a card or board game instead) there may be other places better suited for those discussions. If your interest in your ideas and designs are centered more around the world-crafting and story elements, then perhaps a creative-writing forum might be better suited -- note that there's a Writing sub-forum here as well.

In any case, good luck finding what you're after.

If you like game design but can't execute on your ideas, have you contemplated trying board game design? There's a lot of similarities (video game design university programs typically start you off with board game design after all) and prototyping your ideas is a lot cheaper and less time consuming. :)

I don't sadly know offhand of a good board game design forum, but if you do happen to look for one and find a decent one, let us know. I spend waaaay too much of my free time thinking about dice mechanics and could use an outlet. Or that's the complete opposite of what I need. Unsure.