Do you quit all running applications before shutting down? I know you shouldn't have to but on a couple of ocassions I've had Mail.app running and it seems to get stuck - Quit on the Mail menu is greyed out and all the time it's running, the Mac refuses to shut down. Only option is then to do a Force Quit from the system menu to kill it off, at which point the system shuts down ok - so check you haven't got some rogue application running that's causing the shutdown problem.
Wonder also if it's worth downloading Onyx (freeware) and letting it do it's S.M.A.R.T status check and it's Startup Volume check, just to make sure things are ok?
The WiFi problem is a bit more tricky as it can be erratic for apparently no reason. Could be that you're getting interference and that your friends (older) MacBook is better at handling it. Wonder if it's worth trying to change channels on your wireless router to see if it helps?
Might save you a trip back to the store - good luck
