I don't agree with making IV's mandatory for several reasons. First and foremost, some people can't grasp the calcs, as stupid as that sounds, but hey, we've all seen what sort of noobs come in trading here. Secondly, not everybody keeps record of the IV's and EV's of every Pokemon they train and over a long period of time they are forgotten. This sucks for a lot of traders as this just isn't good enough, but there are traders out there who don't care about IV's, and there's no reason that they should not be able to make a deal for the POkemon because a rule here stops the thread owner from posting it.
However, I do agree that for all ready-bred Pokemon posted such as shinies and events, natures should be mandatory. I know from my own experiences that if I head into a trade thread and the natures aren't posted I automatically leave because the hassle of finding out is, well, a hassle, and I can only assume that is the same for others. And in the case of natures, there is no excuse not to post the, after all, if you are able to type you are able to read.