1

I'd probably cut out Yunnan, and extend Xiahé - Labrang and Yangshuo - go to see the rice terraces and perhaps Chengyang Bridge as well.

2

I disagree. Cut one day out of Pingyao--it's small and you don't need that much time there. I would omit Xiahe/Labrang. I've just come from there and 2 full days is enough. The problem is, it takes a bit of work and time to get there, and it is quite out of your way, considering everything else you have. Xiahe is much better bundled with a trip that explores the Amdo area of Qinghai/Gansu/Sichuan. I think that time is much better spent in Yunnan, but not in Yuanyang since you can do Longsheng Rice Terraces near Guilin. My suggestion:
Days 1-4 Beijing including Great Wall
Day 5--travel to Pingyao, Pingyao
Day 6--Pingyao
Day 7 travel to Xi'an (don't bet on getting a train sleeper by the way), balance day in Xi'an
Days 8-9 Xi'an
Day 10 fly to Guilin transfer to Yangshuo
Days 11-15 Yangshuo and Rice Terraces areas
Day 16- train to Kunming, Kunming
Day 17-24 Yunnan to be determined--I'd suggest choosing from Dali, Lijiang, Zhongdian/Shangri-la maybe further north near Deqin. Not all of them though, 8 days is not a lot for Yunnan. Fly via Kunming to HK.
Days 25-27 Hong Kong (3 days is enough, and HK is expensive)
Day 28 Return to Beijing.

Remember that if you only have a single entry Chinese visa, once you leave the PRC and enter HK, that visa becomes void. If you need to return to Beijing for your flight home, you would either need to get another visa, or schedule a flight from HK to Beijing that arrives within 24 hours of the departure of your intl flight out of Beijing to home--so you can use Transit Without Visa. If you have a double-entry Chinese visa, you're all set and can schedule your HK-Beijing travel air or overland at will.

3

Thanks so much for the replies, drumbrake and jiejie!!!