Ten Things I won't allow in my Universe, by Henry J. Cobb

10: Reactionless space drives.

Goodbye conservation of energy. Hello near-c rocks. Splat!

9: Matter to energy conversion.

Hello near-c rocks. Splat!

8: To anywhere teleportation.

I'm over your cities, killing your men.

7: FTL travel.

I'm in your Past Light Cone, killing your grandparents.

6: High trust ion engines.

The concept of all of those densely packed ions is simply repulsive.

5: Plasma weapons.

Even more repulsive.

4: Laser weapons.

Zap, you're dead as soon as you show up.

3: Portable fusion power.

If fusion was easy, the sun would explode.

2: Space fighters.

Useless against real warships and inferior to one shot or multiuse missiles.

1: Space warships.

Without the above they're too fragile to survive if they carry a reasonable fuel fraction. Plus, if there was anybody for them to fight, then why aren't they here already?