Tailrider: I can fit a shirt, pants, belt, socks, soap and wash cloth, and a sandwich into the main pocket but it's a tight squeeze. Having the sandwich in a plastic container helps keep it from taking the shape of your clothes. I have some bandages, my work badge, garage door opener, and deordorant in the side pockets. I carry my emergency tools in a seatbag. When I need to bring more in, I use my Bug along with the Tailrider.
Wheels: You can get a set of 32 spoke Mavic Open Pros for under $200 if you look around. They are quite light for the money and will help if you have a lot of uphills (assuming your stock wheels are heavy). You won't give up much strength either although I wouldn't want to do a loaded tour with them.
Aerobars: Never used them but those who have say they work. If your ride has long sections without traffic, they could be useful. I wouldn't want to give up that much control while getting buzzed by 5 cars in a row (like this morning's commute).
Fenders: What's a little rain when it's as warm as it is in AZ? Suck it up
Other ideas: I know it's mostly the engine, but on my lightweight roadbike, I'm a good 2-4mph faster uphill than on my commuter. Maybe it's just because I only ride the lightweight bike on nice days for fun rides (it's too nice to commute on) so I've got more energy. Regardless, less weight is always a plus if it fits into your budget but don't go crazy over it. Once you start caring about 50 grams, you've gone too far.
I've done a 14 mile commute (for a week) on my current MTB (rigid fork, slicks, fenders, rack, etc.) and even with a lot of hills it's not painfully slow. But, if I had a 20 mile commute every day, I think I'd always be using a light road bike.