I used to be vehemently opposed to deleting our worlds... because there is some really cool stuff that would be sad to see gone... But I must admit, my thoughts are changing...
I also never thought I'd want to play on survival.... the thought of not being able to go out at night, having your stuff exploded, and being killed and having to run back and collect your stuff seemed completely unfun to me..... but now I seem to spend more time there than I do on creative..... Partly because there aren't many people about on creative... partly because that extra challenge of potential death is kinda fun, and actually needing to grow wheat for a purpose and stuff makes for more depth..... but mostly I suppose it's because we've had such big patches of being on the temp servers, and I didn't feel inspired to build much on temp, but now the "proper" worlds are back, I've lost the momentum I had with my projects... I remember trying to mark off a border for a grand new extension to my land on creative... but now I can't remember what I'd planned to do there

(and it can't be for a giant AT-AT.. as that would need to be in the snow

) Much as I'd hate to see everything on there go, I no longer think it would be a bad thing to start again (with a smaller world) on creative...
(don't kill me Bazz

)
Which is strange for me, as I'm usually one to keep tinkering with things to infinity because I'm never satisfied they are "finished" yet... So I agree, I do get more enjoyment out of working on stuff and once it's done then I'm a bit lost as to what to do next.... other than to just go off and build something else... just to have something to do... and leave a bunch of buildings in your wake that you'll never go back to.... and have to travel out further and further to find more space to build/mine... But then again I suppose that's the main purpose of the game - build something, then go build something else - just because you can... and it's fun
The problem with just clearing the slate each month or so though would be that we'd have the same "temporary" feel that we had on the temp servers.... there is no point building anything elaborate because it's all going to go.. so then what? do you even bother? The permanency of the world is what makes you want to build something time consuming, because you know you'll be able to do it. Ultimately I agree it would be nice to have a clean slate and the excitement of starting new.... but I think it would be unpopular and probably not good for our division.
See, if we'd thought about it before starting the SMP, we could have sectioned off a large part of the world, and declared that to be a temporary space where the chunks get deleted every so often, so that we have the best of both worlds. I don't know how easy that would be to achieve now though, given that people tend to spread out in all directions...