Yes this idea came to me when typing in another thread. Each colony would gain a happiness rating. There would be 11 tiers of happiness all colonies starting an Neutral.

How will this effect the planet? It could do two things.

Improve or reduce the production of a planet (from -100% to +100%). (Note: This could also be a modifier for population.)
Improve the chances of someone invading or reduce them (-100 to +100 invasion defence).

The decay rate should be prety slow if nothing is done to maintain the planet. Like it drops down to the next category every 2 weeks to every week.

Though could be buildings and artifacts that could help improve or weeken the Happiness rating. Even using energy and creds to hold an event to improve the rating of the planet (once per day per planet or some such).

check my tax thread redlaw

I wouldn't want the game to be where it's to micro managed. But a feature like this around this concept might be pretty cool.

mebbe its could be a slider for percent of tax
and just a tech tree for happiness structures
thats not too micromanaged
the hard parts is on the back end adding code to adjust for colony happiness level in defense/production levels

of course there should also be a potential for a planet to break away from your iron fisted rule if they stay under 50% happy for too long that would keep peeps from overtaxing there citizens just to get the extra credits and then also the playable character selections would also have to have tolerance to taxation levels added to each different species

Oh that would be awesome. Then I could get rid of some of these planets I don't want instead of paying galaxy points. =D

I think instead of breaking away maybe the planet would send out a distress signal to alert your worst enemies to attack your butt. Even though they have been plundering you for weeks now. It would give them another reason to keep attacking you. To free the helpless planets you keep under an iron fist! =))