When I was working on my undergrad degree, I took a course in physical anthropology, the lab of which was osteology (nothing but old bones). I remember a similar question came up and the professor said that morphological differences b/w the australopithecines and H. habilis were insignificant. The reason for the taxonomic difference was the presence of stone tools at habilis sites and lack of them at australopithecine sites. Now it's not so cut-and-dried as it sounds, but it speaks to the point made by another poster that evolution isnt black and white but rather undulating shades of beige and grey. Which is exactly what you'd expect from a process that tinkers, gerry-rigs, makes-do, and sometimes transitions slowly (and sometimes leaps). Creation, by contrast, coming from a perfect God Designer, predicts that things SHOULD be cut-and-dry, easily demarcated, discrete entities or kinds and no relation whatsoever b/w--uh, "basic kinds," whatever that is.
One of the key differences between (modern) ape and human lineages, however, is bipedalism, indicated by the location of the foramen magnum, or the big hole in the skull where the spine engages. Apes' are located toward the back of the skull while hominids (the human line) are directly anterior. It's worth noting that all the skulls in TD's line-up have an anterior foramen magnum, meaning that they are all hominids, though not necessarily directly antecedent to modern humans.
Complicated? You bet. That's why the strawman arguments of creationists are so insidious, and why there's so much misunderstanding, which is capitalized on by creationists like the WT. I sat through the WT this past Sunday and honest to God, lost a half-dozen IQ points. I kept looking around, thinking to myself, "C'mon, folks, is there even a teaspons of critical thinking being applied anywhere in the room?"