Well this is exactly why there is so much confusion on this subject. If I asked "what is evolution" I would get several different responses. I would get your response, which is perfectly reasonable. Then I would get the Natural Selection answer saying that evolution occurs in response to catastrophic events, and only those species that adapt and survive are able to live. And then I would get the genetic drift answer saying that natural selection isn't necessary for a species to evolve.
The main reason there is so much argument on this subject is because everyone is arguing about something different! Personally my opinion on Darwinian evolution (as it's spelled out in his book) is that I'm skeptical. I'm not willing to say evolution doesn't happen because that's absurd. but at the same time I'm not willing to say that everything happened due to random chance because that, IMO, is equally absurd.
Can we get a unified theory of speciation? Is that too much to ask? :)
Actually, we now believe that all of the mechanisms you mentioned may occur. Is it unreasonable that there are different mechanisms that may contribute to a complicated process?
I'd rather doubt it. My guess is that there are many methods of speciation. Both genetic drift and radical mutation are possible and I think probable.
Also valid I think are Evolutionary Sub-theories that deal with species equilibrium with punctuated radical changes brought about by large meteors, Human intervention, etc.
It may seem that everyone is arguing something different, but that's mainly due to the wide range of attacks by anti-evolutionists. As stremba posted before, evolution can be driven by many mechanisms such as genetic drift or catastrophic events.
As far as speciation, it is pretty well understood. Evolution predicts that speciation will be muddy and difficult to observe. The misunderstanding of this point is why anti-evolutionist insist upon observing an ape giving birth to a human.
You noticed; eh?