I believe that the reason new TLDs have been slow to take off cannot simply be explained by the fact that some of them have more than three letters. It's a matter of habit and the marketing dollars that have been put into .com and to a lesser extent .net and .org. However, different gTLDs have an equal opportunity to rank well in the search engines as their traditional counterparts. Hopefully, particular new TLDs (which didn't have a flawed mandate from the beginning) will also gain better recognition and identification in the average web user's mind over time as people build useful websites on those namespaces.