Asteroid Belts lower the accuracy of your ships (the penalty is 20%), hence lowering their overall damage output. It cuts both ways, and the only way to mitigate this penalty is through the Akkan's Targeting Uplink.

Gas Giants have an Explosive Gas Pockets ability where if a ship in the gravity well is destroyed the resulting explosion damages nearby ships (I think the damage is like 80). Think of it as everything in the gravity well being afflicted with the Kortul's Volatile Nanites (without the damage reduction penalty and lower explosion damage).

Thanks--I just haven't had it happen yet.  The damage reduction I did know but I thought you could collide with asteroids as well.

I haven't noticed the gas giant damage even when close...is it automatic or random?

It would be a lot more useful and fun if it affected ships passing through.  One solution would be have two effects for a gas giant...the grav well effected in a torus around it with one effect and  the more severe one inside the torus close to the planet.  If there was a risk/advantage to each it might bring them into play more often.  Maybe the damage reduction in the well except where the explosive damage area occurs.

Maybe reduce damage by 20-30, then have it effect everything in the Grav Well.  Or let the damage stay as it is, but be effected by mitigation(My Preference).

-Twilight Storm

The Distant Stars mod more or less addresses the whole 'boring sectors' problem.  You may want to give that a try if you're looking for some enhancements to Sins without drastically changing the core game.