What you have to remember is that when the King of Chin (Qin) undertook the conquest of China, he used genocide as one of his weapons.
When he was done he'd changed the ethnic face of China forever.
The Hakka still don't look all that much like the Han.
"The "theory" on Hun origin of Hakka was based on very fragmentary > blood typing and DNA analysis > done by Japanese and Russian researchers. Undoubtedly, the > original Hakkas migrated under the > pressure of the northern intruders. During this cultural conflict, > there could be a very small portion of > the population derived from voluntary or involuntary marriages > causing the inclusion of non-Han > factors. However, using the isolated examples to support the > Hun-Hakka theory is ungrounded."
That's a gross generalization - that "Hakka still don't look all that much like the Han."
In fact, "Han" people don't look like each other. "Han" Chinese is made up of many different ethnic groups essentially. Han Chinese in Sichuan will look vastly different than Han Chinese in Shandong. And Hakka, who has migrated south, look more like Han Chinese in the south (which are again, influenced by Viet, Hmong, etc.)
The skin tones of Han is also quite different. The facial features are also quite different amongst itself.
My dad, for example, came from Jiangsu province. He has incredibly dark skin and high bridge nose. My mom, from Taiwan, has much lighter skin and flat nose. My wife, from Shandong province, is probably whiter than most caucasians (though there is a debate amongst their family of whether she has 1/32 Germanic blood) and features that are drastically different than other places -- She's 5'8", not really short for a woman in China. Her hair is not dark "black", but actually much closer to dark red. Her facial features is very much "Shandong"-like, the perfect example would be the actress Gong Li from Shandong province (very light skinned, tall bridge nose, etc.)
I'd have to say that "Han" chinese is more about a culture than ethnicity. Northern Han Chinese is vastly different than Southern Chinese, ethnically speaking. And sometimes those rules don't apply -- my dad has skin colors that will resemble someone from Guandong province or Vietnam (i.e. darker skinned) but he has facial features from northern Chinese.
In reality, Han Chinese is probably one of the biggest melting pots there is, ethnically speaking. Culturally it's fairly unison with regional/local differences.
As far as Hakka goes, i have met Hakkas that are dark skinned and Hakkas that are light skins and look more like Japanese (flat facial features). They do speak and proud of their own spoken laguage, but that's about it, ethnically speaking they're not much different than the Hans.