A lot of good thoughts stated so far in this thread. SIRT (Steve) did a terrific job explaining and opining on what weathering means to him. I would place myself in that same school of thought on the subject. The huge difference is he's one of the true masters at actually DOING it...while I'm just getting started!! But for what it's worth...
I first dipped my toes in the weathering water about a year and a half to two years ago...and now I'm hooked. So much so that when I build my first "good-sized" layout (hopefully will get started later this year), I envision weathering the entire thing...sort of along SIRT's line of thinking. Ideally everything on my layout will look like (or as close as I can make them look like!) what I remember seeing growing up in northeastern PA.
I've got a long way to go to achieve the results that will really make me happy with my work; however, I've learned a tremendous amount in a relatively short period of time just by taking a "DO IT/TRY IT" approach. What's really cool is that items I thought looked good two years ago now look crappy to me. I guess that's the definition of making progress toward a goal!! Here are a few photos of an item I've weathered...
BEFORE:
AFTER:
