OK, I've been updated to a new bike and here are some notes:

1. The recommendations on tires have changed quite a bit. I used to ride 23mm at 110 psi, but now folks are recommending a wider 25mm at 90 psi, apparently it's about the same footprint on the road and much more stable.
2. Heart rate monitors. Wow these have really improved, while you can still wear a chest strap (they have essentially infinite battery life), the new Scosche Rhythem+ is just \$80 and has a simple rechargeable battery that lasts 8 hours. Way more comfortable to have an arm band than something that is around your chest.
3. Power meters. DC Rainmaker was right, the Powertap P1 pedals are expensive, but boy are they easy to setup and they just use a standard lithium AAA battery. Setup with the old Garmin 800 was  a snap.
4. Shimano automatic shifting, well the guys in the peloton don't need it, but with a new battery and a firmware update. You do need a Windows machine to do this, now you just hit the right paddles to go up and down the gears and the front derailleur shifts automatically for you. Nice and no more cross chaining.
5. Be careful how you leave your bike, if you accidentally leave the shift lever against a wall, it will drain the battery of the Di2. You are warned 🙂
6. While most folks still use water bottles, I find the trusty dusty Camelbak Mule still more convenient and probably more aero, plus your bike looks way cooler without cages.

Things left to do:

1. Shimano D-fly Di2 transmitter. The latest firmware let's you install wireless transmitter that speaks ANT+ and Bluetooth, so the shifting system can tell your bike computer what gear you are in. It's a little complicated to get the parts, but Shimano has a good list. The EW-WU101 is the variant that connects at the rear derailleur, you take it from basically hook it inline. This works because the two connectors are on the same side. The EW-WU111 is for internal mounting inside the frame so the connectors are opposite each other. You also need a newer battery the DN101 because it has the firmware to manage this and it needs more memory.
2. New maps for the Garmin 800, turns out you don't have to pay for maps, you can use Open Street Maps. It's a little clumsy, but DC Rainmaker describes it pretty well, someone is running a server which scrapes and reformats Open Street Maps, go to Garmin.openstreetmap.nl and select bicycle map, then choose the segments you want. You will get an email later with the location of a huge file. You then copy the DMG into your SD card in the Garmin directory and you should have it on your Garmin 800 at least.
3. Cycle computer. You do need a new Garmin like the 810 to actually see the data however. Not surprising since the Garmin 800 is eight years old, but it does work pretty well.