I have been looking into a few things about the drive train, specifically rear end gearing lately. I did a search and found a few things on this subject but was wanting to kind of get a thread with all the inforamtion in one area.
What do 4.10/4.11 gears really do for your car? Or far that matter what does any gear change do for your car?
They increase the mechanical advantage so basically the engine rev's more to move the car less but since it's easier to do, it moves quick. Think changing gears on a 10 speed except that your engine can pedal faster. 
Does having a bigger cam in your car make any difference in the gears you select?
Possibly. Depending on the cam you choose and you purposes it will move the power band. If you have a cam that brings it in the top end you'll run out of gear when the powers coming on. On the lower end with large amounts of power (HP) you'll roast the tires without trying and have no grip.
Does increasing or decreasing the gear ratio in the rear end add added stress on the rear end resulting in other additions? Basically if you add gears should you change other things when you do this?
Since switching to a lower gear ratio INCREASES the amount of power that is applied to the wheels then yes. Beefing things up does help, but mostly if it's a manual just make sure you don't abuse it.
Does a gear change affect drivability, fuel mileage, longevity etc.....
If set up correctly longevity shouldn't be much of a problem. Fuel mileage will be affected since it takes more rev's to move the same distance, however depending on your habits since you can drive in a taller gear some have seen improvements. Highway mileage will definitely take a small hit. Some gear noise might be heard.
I have seen a few threads about problems with gear swaps and a variety of opinion for the failures, so when doing a gear swap what should be looked at when doing this, what should you be aware of when considering this mod...i.e. Pro's and Con's
It's best to think what you want to get to in the end. If it's a 1000hp screamer 4.10's might be a bad idea. Make sure you take it to a place that has done it before and had good results. JRE has done quite a few as well as some other good shops on here. LPE offers a 3.91 gear that while pricey seems to be beefier than the 4.10's out there while offering similar gains.
Vendors, Experts, all general opinions welcomed. Do not really want this to turn into Vendor A is better than Vendor B type or Product A is better than Product B.