I don't really like our feather and down duvet any more, it just feels too heavy and either sweaty hot or too chilly. I've never been a huge fan of synthetic either (fussy).
I see about wool duvets and silk duvets, they are supposed to be temperature regulating. Is that actually the case or just marketing guff? DH and I always disagree on when to change the duvet (got a 4 seasons one, annoys me even more as the two layers move around in the cover). Obviously they are really expensive, so I don't want to get one if it's not going to be brilliant.