I've completed the main quest, and a number of side quests as well. One thing I wondered about. . . there are a few ways in which your character changes the world, but I really think the devs passed up a cool opportunity to show greater change as a result of character actions (although, since I'm still pretty far from completing all the side-quests, I think, there may be more ways than I yet realize).

But, what I'm thinking of is this. . . to some extent, what I'm about to say depends on character choices you make while playing, but overall, as you go through the game, and particularly if you continue along the Brotherhood of Steel quests after the main Project Purity story arc is finished, you have a tendency to greatly improve the security situation for residents of the wasteland. Additionally, of course, you create a very large supply of clean, un-radioactive water. After these things, wouldn't you think you'd begin to see things like people planting small gardens (both vegetable gardens, as well as ornamental/botanical gardens). As security improves, wouldn't people begin to start small industries (metal recyclers/smiths, brickmaking, etc). Wouldn't you expect the towns to start to develop a little bit - construction and architecture improving? Wouldn't you expect them to start to clear the rubble, and tear down some of the more decrepit small buildings, so that if nothing else, not every town looks like a complete disaster forever?

I mean, it seems like the main hinderances to 'progress' in the wasteland are the combination of environmental pollution, coupled with constant threats from raiders/slavers/monsters/enclave, etc.

Seems like with fresh water, and improved security from the Brotherhood of Steel (who, now that you've exposed the Super Mutie breeding grounds to them, as part of the main story arc, ought to be able to go in and clear out their 'nest', and break the cycle of Super Mutant creation, causing the SM's to become about extinct within a few months or a year or so), it should be possible for small changes to start occuring throughout the wastes.

I just think it would have been totally cool if the world began to slowly change in subtle ways, as a result of the things I was doing - I wouldn't expect any huge changes, but just small signs of progress which mostly don't seem to be happening?

I did an indescribebly good guy, and the only change I noticed was 3 Dog said good stuff about you on the radio.

But stuff should change......................................But it dosen't so that is not cool.

It could be rads still for the crops, but towns would improve.

Problem is that making world changes is very hard and takes a lot of work who could be spends on other thing like 20 or more quests. Broken steel actually did a decent work here with the water distribution.
One classical way to solve this is to end the game and show a video of how things changes, not an option in a open ended game.

You can make small or moderate changes here and there: save/destroy several settlements, for instance. And you can help or hurt individual characters in Rivet City. Like give Mr. Lopez a reason to live, or help him kill himself. Or, bring Angela and Diego together, or terminate their relationship once and for all.

I hope you realize that developing the game Fallout 3 in it's current form was a tremendous amount of work. I mean a huge task to undertake. Does that mean it wasn't worth doing? Not at all. Hard work pays off. I'm not going to complain if a developer wants to take the extra time and resources to really polish their game and get it as good as they possible can.

It is merely hinted that things might go for the better in the long run, in the settlements you help. The changes in Fallout 3 are much more long-term, and even the main storyline never tells you whether the Brotherhood managed to make things right or not.