Every once and a while I come across beers that say they can be kept up to 5 years and become more developed over that time period, while other beers that are of the same style don't indicate this at all. I know that some styles are defined by how long they can be cellared, but for other styles how do you know? So my main question is:

What are the main characteristics that differentiate normal cellaring period beers from beers with extended cellaring periods? Does it have to do with the amount of hops? The amount of alcohol? The type of yeast?

EDIT: In retrospect, this looks like it would be a good wiki question since it's more of an information gathering topic than a personal, concise question.

Here is a compiled list of comments made below as well as answers found elsewhere:
Please feel free to edit this list (it is a wiki).

These are the characteristics that people have associated with good cellaring beers:

• Wild yeast beers
• High alcohol content
• Proper storage throughout the lifetime of the beer

Note: The lack of a characteristic does not imply that the beer would not be a good candidate, for example wild yeast beers, such as lambics, do not usually also have high alcohol content.

Styles of beers that lend themselves to cellaring:

• Lambic
• Barleywine
• Oud Bruin
• Belgian Ale

Characteristics that can develop throughout cellaring:

• Floral hop character diminishes
• Hop bitterness diminishes, but at a much slower rate
• Flavor complexity is developed
• Malt character is developed
• Can develop sherry and port-like characteristics

Keep these characteristics in mind when selecting or designing a beer that you wish to cellar for an extended period of time.