1}When the name is a) composed of popular search terms that b) accurately describe the idea and c) there are no alternative terms, available, that can express the idea as well as, or better than the longer term. NewYorkRealEstate is an excellent example. "New York" and "Real Estate" are the top, keyword search terms for expressing the idea. Shorter, alternative, names are available but their search terms are weaker, ie "NY" Real Estate, or they are compsed of search terms that are weaker and that are more limiting/confining and/or less accurate. ie New York "Property". This is not to say that NewYorkLand or NYLandSales wouldn't, both, be very good names and a more accurate description of a New York company that specializes in the sales of parcels of land, but NewYorkRealEstate would get the company a heck of alot more hits.
2)A longer term that a)expresses a product or service in a unique, site-specific manner and that is, often, b) a common and/or catchy phrase. ie TheOneStopPhoneShop. From a resellers, standpoint, registering these names can present a problem because there are several of them out there and lining up a company that wants to or is willing to market themselves this way and that has not, already, registered the name can be difficult, and may require alot of persuasion and/or luck if a sale is to be realized.
3) A work around. Many companies, either can't, cannot justify, or, simply, will not shell out the money for a site that uses the shorter, preferable term and will lengthen it to avoid the expense. This has been a growing trend in the, (generally, short lived) sites that motion picture companies use to promote a film. ie [Name]theMovie, TheOfficial[Name]Site, etc.
4) A descriptive name that is composed of the slogan being used in a promotional campaign and that is, genrally, backed up by TV and radio spots,, newspaper and magazine ads, etc. These names are, also, a hit and miss proposition for the reseller.
So, to answer your question, if reselling is your intent, and not development, go with #1 and you can "get away" w/ #2,3 and 4 but, do so, at your own risk.