Texas Flyer:
Although it is fair to say that China Southern probably does not have the same service and safety standards as some of DL's other partners, I would be surprised if China Southern was worse in these regards than other Chinese carriers. The flip side of the coin is that this new route be Delta's only route to China. In order to do this, Delta would have had to do itself (unlikely) or code-share with another Chinese airline - probably about the same standard as China Southern. So, although many people may not want to fly this new route, as least it's another opportunity for those of us who want to explore different countries and areas of the world that US and European airlines do not fly to - namely Guangzhou. It should also be noted that some of Delta's existing partners do not have particularly high service and safety standards and could qualify to be "third-tier carriers" as you put it. These include Malev Hungarian Airlines - code-share JFK --> Bucharest and Frankfurt --> Bucharest. Also, Delta Air Lines recently signed an agreement with a small Venezuelan airline that I can not remember the name of. Most Latin American airlines - with the possible exceptions of Varig, Avianca and Lan Chile - are notorious for not having very high safety standards. Delta's current Asian partner, Korean Air, is not known for its high safety standards.

So, I think it's important to view this new route as a new opportunity for travellers to reach south-eastern China.