I'm hoping some of you bike mechanics can explain this to me, at least in general terms. Everyone seems to "know" that some compnnents are better than others, but no one ever says what, exactly, makes them better. What makes the Shimano Deore better than the Shimano Alivio? What, besides the name on the box, makes a SRAM 7.0 better than a SRAM 5.0?

One guy (who may not know what he's talking about ) told me that the lesser models have plastic bearings, while the more expensive models have metal bearings. Is he right?

I realize that there might be MANY differences from one brand/model to another--but is there a general pattern? Before I spend an extra $100 or whatever for a higher level of component, I'd like to know why I'm spending the money. (I don't want to be one of those guys who brags about his components without having any idea what he's talking about.)