The difference is on average humans have the ability to plan, use tools, and effectively modify our environment.
It's almost certain you can't separate chimps from humans this way. Chimps not only use tools, they *learn* to use certain things as tools and the knowledge spreads between chimpanzee groups through individuals -- in other words they have a rudimentary technological culture.
Chimpanzee groups engage in warfare to annex territory, and it's not just a case of encountering other groups and spontaneous fights breaking out. They *invade* the territory of other groups. Surely that shows rudimentary planning. Within a group there is politics. The dominant male is not necessarily the strongest; a clever male can defeat a strong one by forming alliances.
Psychological experiments support the notion that chimps have a consciousness of self. Chimps have been taught American Sign Language, and appear to use all the cognitive features of language. Objections have been raised that this is just operant conditioning, but the same objections would apply to human use of language.
A hundred years ago, the idea that chimps might be persons from the point of view of ethics would be ridiculous. They were just animals in the forest. But a century of research has seriously undermined nearly every substantive distinction between humans and chimps. At this point the verifiable differences between chimps and humans aren't ones of *kind*, but of *degree*. Chimps use tools, but simpler ones than humans do. Chimps can use human language, even learn it spontaneously, but their vocabulary is in the hundreds of words, not thousands for a fluent human speaker.
If there is a defensible *ethical* distinction between the status of chimps and the status of humans, that distinction ought to arise out of clear-cut differences between humans and apes. At present there are only two clear-cut distinctions between humans and chimps. The first is genetics; chimps are close, but past attempts to create human/chimp hybrid have failed. Second, humans *rely* upon our advanced behavioral capabilities to survive. Tools are useful to chimps, but *essential* for us. Yet it is hard for me to see how we get from "chimps can get along without tools" to "it is immoral to experiment on chimps." One doesn't follow from the other.
If the answer is "well, they just aren't *human*," that has implications which are nearly as counter-intuitive as the notion that chimps have some of the same rights as humans. Most people would assume that if we ever met an alien, non-human civilization made up of self-conscious individuals, that hunting those individuals for pleasure would be morally wrong, and perhaps legally impermissible because while not human, they are "natural persons" with at least some of the basic rights of humans. Furthermore, if genetic tribalism is the ethical basis of law, why not favor Europeans over Africans, or vice versa?