Rod Weight Considerations
Start off by understanding that everyone has their own opinion. Here's mine, so you know my bias, I fish 80% trout, 10 % bass, 10% panfish.
Your rod serves several different purposes - casting, manipulating the fly, hooking, and then fighting the fish.
A heavier rod is going to encourage you to cast farther, depending on what you are fishing for, in some situations, that's good, in many/most it's bad. Particularly for stream trout, most of the people I see who aren't catching fish are fishing too far away. It effects the way the fly is fished, see next paragraph.
With perhaps the exception of a bass bug/large fly, most flys are better manipulated with a lighter rod. In most stream trout circumstances, the way you manipulate your fly may be more important than the pattern you choose. I fish the same pattern 85% of the time, early spring thru late fall. It's very impressionistic, and the way it's manipulated is what makes it effective. There are some match the hatch situations, but those tend to be the exception rather than the rule. Heavy rods tend to make subtle manipulations difficult if not impossible, they are effectively too stiff.
Again with the exceptions of large flys, most hook ups are better accomplished with a lighter rod. Again a heavier rod will often jerk the fly out of the fishes mouth. Trout flys don't take much to penetrate the fish.
Again with the exception of LARGE fish, a lighter rod fights the fish just as effectively, sometimes more so, than a heavier rod.
