What they never seem to get is that since the process is so glacially slow, the only way to "observe" it is by backtracking to the most simple life forms, at which point the whole process becomes elegantly simple to understand, and all sorts of things (like vestigial legs, tails, etc, or the fact that our bodies are better designed for a quatruped rather than bipedal animal) all fall into place. And that's not even getting into how all the genetics fit together.
All they can see is that no new species magically arise in front of their eyes.
But the thing is that the world is so full of so many highly complex life forms with their own various niches and survival strategies that even if a new species WAS to be "in progress", so to speak, no one would ever live long enough to see it emerge. The only way to see that it did, again, would be to backtrack existent species and identify where it had branched off. And in the meantime, the only changes we can observe are those of the "micro" variety.
Sadly, the only really new things which might come about at this point would likely be those which found ways to take advantage of all the new ecologies and environments which would open up once humans had gone extinct.