First off, solo threads rarely have an OOC thread - that's typically done with PMs, IMs, or emails (or on rare occasions, it's handled in-thread with typographical distinctions).
Secondly, some of the separations are done for the benefit of people browsing the RPs. Aside from the various levels of kink (which are hopefully understandable in their separation), the distinction between human and exotic (which includes hermaphrodites and furries) has been known to raise strong feelings. A number of people who come to Elliquiy are pleasantly surprised that furries aren't ridiculed here, like they are on other RP forums.
Big Groups are separated from Small Groups, due to the fact that it's harder to manage a Group game that has reached 'critical mass', and often consists of half a dozen or more threads. The limited number of threads in the Big Groups portion of any section minimizes the chance that a stray thread will get 'lost', and the individual subforums for the well-established games further contain them.