I've put in my two cents before, as I have already been quoted. I'll wrap up some points.
1. The non-existence of the human species in bio/geo PoD scenarios. This is not always true, obviously, with a recent or restricted PoD. And even if not, I think there should be some room allowed. If we want to explore a bio/geo PoD, and that bio/geo PoD doesn't explicitly and directly prevent the rise of humanity, then we should say ceteris paribus and run with it. As soon as there is a quantifiable effect on human development that's more than just the blowback from butterfly effects, it should be taken into account. Like in Ian's example, an island Africa PoD wouldn't necessarily prevent the rise of humanity (even though the butterfly logic says it might), but it will effect humanity's migration patterns, and those will have effects. A more larger example, if there is a continent in the Pacific then you can say that the odds of humanity rising unchanged is minute. But the odds are not zero, and it's the only scenario where we're not making up things whole cloth, so I don't see why we can't run with it until the direct effects on Polynesian or European explorers occur.
2. As for biological and geological deterministic and human behavior not, well, that can be argued. But it is true that these kinds of PoDs require a higher level of rigorous fact-checking and justification to retain plausibility and suspension of disbelief.
3. The only reason there's not a designated forum for these topics is server considerations, I believe. But I maintain that "What if there was no Panama Isthmus?" or something along those lines deserves a place here more than with scenarios that deal with supernatural or extraterrestrial intervention, often of the fictional kind.
4. In general, they tend to be ignored or accepted in practice. Lands of Red and Gold is a biological PoD, for instance. Due rigor and good writing will normally let you get away with anything that isn't blatantly impossible.