To be honest, there's nothing here that's not either been said before or fairly obvious, but here are my comments:
1. Get South African currency before you go. The Namibian dollar is pegged to the SA rand and is accepted everywhere. Saves finding ATMs (though to be fair, this wasn't difficult).
2. Bring a spare battery for your camera. Charging facilities are available (both on the bus and at every campsite), but it's good to be cautious!
3. I found a pillow case was handy to stuff my fleece and other clothes items in to make up a pillow. Hardened campers might not need one, but it helped me sleep!
4. The adapter you need is the 3 round pins that are marketed for South Africa.
5. The water at every camp site was fine. Many people did buy bottled water (and there were plenty of supermarket stops to stock up on supplies) but I didn't. I reckon that at most campsites it taster better than my water back in the UK.
6. A notebook can be a good way of keeping track of all the species you see, if that's your thing.
7. Don't bother booking an additional game drive. We saw well over 100+ species from the van and the waterhole at the first Etosha campsite is floodlit; we saw 26 elephant at the waterhole simultaneously one night and then 6 rhino the next.
8. I didn't think the books that exodus 'recommended' were all that great. The van has 7 or 8 reference books (birds, mammals, insects, flowers, geology etc etc) so personally, I wouldn't buy any of the books they list and just get a general guide instead, such as the 'Insight Guide' of Namibia (10.99 on amazon), which was very very good. Highly recommended for general information on the areas you visit.
9. The flight via Frankfurt is a bit of a pain, but that's all part of the experience! Just check that your hold luggage is being checked in all the way to Windhoek on the way out, or Heathrow on the way back.
10. I travelled in July/Aug 2013 and we had great weather (about 7-12C at night and 24-28C by day), bar two very windy days at the start at Sesriem and Solitaire. For the most part it was a dry heat, so all you need are plenty of t-shirts, one or two long-sleeved tops and a fleece. I took a down jacket as so many other reviewers said how cold it was but never used it; maybe we were lucky? Anyway, it helped make a good pillow. Zip off trousers were good as it meant it was easy to keep warm in the cooler evenings and mornings. I took walking boots but they really weren't necessary. Sandals were best most of the time.
11. I can wholeheartedly recommend the quad biking whilst in Swakopmund. Great fun! Brad Pitt's been there 13 times, apparently.
12. Ear plugs are good to muffle the sound of any snorers in the group!!!