I am a little unsure what you are asking here, too, breakaway.
Like TheOcculist said, it is because "Hello Kitty" and "Desperate Housewife" marks have
become famous that they are given the most legal protection under trademark law.
A trademark is originally awarded the most trademark protection because it is unique or creative (say, through the use of made-up words like
Kodak or when words are used out of context like
Time magazine) - but a trademark that has become well-known and connected in the public's mind with a product or service can gain the most legal protection under the trademark law over time (like "Ben and Jerry's").
Common or ordinary words ("Sam's Pet Store") or descriptive words ("Rapid Tire Change"), or trademarks that use first or last names ("Pete's Diner") are generally awarded the least protection, unless or until they become well-known ("Dunkin' Doughnuts", "Speedy Printing", "McDonald's").
When words are used out of context or are an unexpected combination of words, these are called "arbitrary marks" ("Milky Way" or "Apple Computers"). Words that suggest what the product or service does are called "suggestive marks" ("SlimFast"). And words that have become connected with a product or service over time are said to have acquired a "secondary meaning" ("Borders").
So, I guess I would say that "Desperate Housewives" is descriptive and "Hello Kitty" is arbitrary and "Sex and the City" is. . . . . . unsure. Perhaps suggestive.
So, does that answer whatever question it is you had?