1. Life Expectancy
The good news is that with advances in technology and science, people are now expected to live longer than ever. The down side of this of course, is that dollars in retirement will have to be spread over 20, 30, possibly even 40 years of your life. See the Census Bureau's life expectancy tables.
2. Rate of Return
Remember that during your retirement years, it is likely you'll want to shift your investments to a more conservative portfolio and thus, you'll probably see lower returns than in your pre-retirement years.
How many of you, over the age of 65, paid more for your last car than you did for your first house? You don’t have to be old or wise to witness the effects of inflation all around. Just look at the price of stamps, gasoline, or college education costs, for a real eye opener. The money you have saved for retirement now might sound like alot, but consider it in terms of future purchasing power.
When budgeting, a lot of people underestimate their expenses in retirement. Although some costs associated with working will decrease (such as taxes, transportation, and savings) other costs will increase (such as travel, hobbies and healthcare.)