Push Reel Mowers are making a "comeback" of sorts, though, in reality they've never really left; they've just gotten better. There are a number of reasons why purchasing a reel mower would make sense such as: better for the environment; more exercise; lighter; and, better for the grass, to name a few. However, a push reel mower is not for everyone nor for every lawn situation.

Below I list 5 areas where a reel mower is not the best solution for everyone

1. Yard Size. Most push reel mowers can cover up to 2500 square feet without too much trouble. If your yard is much larger than that, you may want to consider a powered mower, and perhaps even one that's self-propelled or a riding mower.
2. How often You Mow. If you mow less than once every 2 weeks, then your lawn will be taller than what's manageable with a reel mower; especially in the late-Spring and early-Summer time. Most reel mowers can handle up to 2" fairly well; some can even handle as high as 3" or more. If you routinely wait until your grass is higher than 2" you could be making more work for yourself with a push reel mower.
3. Yard layout. If your yard is on a hilly terrain then it's even more work to go back up the hill on each pass. One way to mitigate this layout would be to cut horizontally instead of vertically, so you don't have to go up the hill each time, just across it.
4. Trees. If you have trees in your yard whose branches cover most of the yard, you may have quite a few fallen twigs, sticks and branches to contend with. That would mean adding the extra work of picking up as many sticks as possible before you begin cutting. Whereas powered mowers can cut through most twigs and small sticks easily; those items will quickly stop a reel mower in it's tracks. Which would add to the total cutting time because you would have to clean the debris from the blades each time they got stuck.
5. Weather conditions. If your grass is wet, it will make cutting a bit more difficult. Depending on just how wet the lawn is, the wheels could simply slide across the grass which would not allow the blades to rotate properly.

Jeff Boyd is the Owner of http://www.reelpushmowers.com, a supplier of Push Reel Mowers and accessories for the environmentally-conscious and health-conscious consumer.