Lars Doucet, forged from the finest Larsium.

I have a lot of opinions about Tower Defense, so I wrote an article about it that is hopefully relevant to your interests:

http://www.fortressofdoors.com/optimizing-tower-defense-for-focus-and-thinking-defenders-quest/

We optimize for two things:
1. Let the player FOCUS
2. Test the player's THINKING

To that end:
1. No Scrolling
2. Total Information
3. Total Time Control
4. Non-Sucky Melee Units
5. Game State is Easy to "Read"
5 1/2. No 3D
6. No Lock & Key Enemies
7. No Freaking AIR UNITS
8. No Physical Challenges
9. No Pointless Variety

See the article for our full reasoning on these matters :)