The biggest mistake I've seen is installing way too much pump for applications like this. The system is normally pressurized by the municipal system, so the circ only needs to move a relatively small volume of water around in a circle. (You only need match the heatloss from the system.) If copper pipe is being used, make sure it gets reamed after cutting. The little lip left on the inside of the pipe by the cutting wheel sets up a nice swirl, and in combination with a constant water flow can erode pipe suprisingly quickly. If the circ is grossly oversized, the combination with un-reamed pipe can be particularly nasty. I've seen pipe fail in as little as 6 months because of this combination.
Remember as well, that you're essentially turning the domestic hot water system into a hot water heating system if the lines are uninsulated. I've seen installations where the uninsulated recirculating domestic hot water system was fighting the air conditioning all summer.
