I've always been a big fan of the traditional aikido no-structure structure. The only variation from that that I've seen and liked is the idea of a 'beginners class' that meets for a number of weeks (8 or 10) and is taught by someone of reasonably high rank and attended by more experienced students who are eager to assist and re-work the basics. I think that it's very important that high ranking students make a point of working with beginners as much as possible, and that a pair of newcomers should almost always be broken up by a pair where both members have achieved kyu rank. Beginners should be encouraged to seek out high ranking members of the dojo and train with them.
The reason I don't like rigidly structuring the class around rank is that I like the idea that we are all 'beginners' in some sense and that we all have to learn from working with each other. It trains in, I think, a healthy respect for and appreciation of beginners that helps them feel really welcomed and taught.