Trademark law, like all civil/case law, includes a lot of gray areas. Trademarks can be very strong, particularly when they are fanciful, coined words such as Kodak, Xerox, Sawzall, Skil, or unusual words, such as Sony or Ryobi. Other, more generic and common words, such as Apple, Delta, Gorilla, or even Craftsman, have lesser protection that is restricted to a narrow category of goods. The purpose of a trademark is to identify a particular seller of goods or services, and the central question in any dispute is always whether the buying public is likely to be confused into thinking that, say, Delta faucets come from Delta Air Lines. In recent years, the doctrine of "dilution" has been added by state statutes, muddying the waters, so that trademark owners can claim that noncompeting, and even noncommercial uses, injure the value of their trademarks.
There has long been tension between trademark restrictions and use of personal names, but I'm sure you realize that a fellow named Tom McDonald is not really free to open up a "McDonald's Restaurant" in his home town. One of my law school professors wrote several law review articles on the right to use one's own surname in commerce.