Originally Posted by
Retro Grouch
When I was selling bikes one of the things I noticed was the tendency of manufacturers to spec rear derailleurs one grade up from the rest of the components on the bike. That's the first part that customer's want to look at. Conversely, my personal experience is that the rear derailleur just may be the most reliable component on a modern bicycle.
The mark of a rookie is the tendency for readjusting derailleur limit screws.
Not sure if that makes me a rookie, but many bikes that come in my shop have poorly set limit and B screws of the RDs, so I make sure to check those when servicing. Especially the "low" (towards the spokes) screw.