One of the greatest ways to add replayability to a game is with randomized maps, creating a different, unpredictable battlefield every time you play. The original X-COM: UFO Defense has random maps; XCOM: Enemy Unknown does not. What's particularly discouraging about that fact is that you can say the same thing about Diablo 2 and Diablo 3, and the staticness of Diablo 3's levels quickly drained the urge to explore.
On the bright side, Firaxis says there will be approximately 80 maps in total, so what it lacks in randomization it will make up for in sheer volume. Plus, we do know that while maps won't be randomized, alien spawn points and alien types will be. So even if you think you've figured out a sure-fire path for your soldiers to take through particular maps, you'll discover plenty of surprises -- surprises itching to send your beloved soldiers to their permadeaths. It's unclear if this limitation will have the same effect on XCOM as it does in Diablo 3 -- unlike in Diablo, where each playthrough follows the same path, you'll never know which map you're walking into. It definitely is a concern, though.