When I first started focusing on outdoor and adventure sports photography, I spent a long time looking for a camera bag I could easily use to capture photographs when I was on the move (e.g. following athletes in the mountains). I've since moved to a more camera specific backpack but at the time I decided my essential requirements were;
It had to be well padded.
It needed to be weatherproof.
It needed to be easy to take the camera out quickly and put it away again when shooting in bad weather.
It had to take my 70-200mm f2.8 Nikon lens
It needed to be comfy to wear for long periods of time
After much Googling about and repeat visits to the camera shops that were then in Edinburgh, plus observing Vancouver-based photographer Andrew Querner using an older model in the climbing DVD, Higher Ground, I settled on a Toploader Pro 75 AW.
What I like about the Toploader Pro 75 AW
Different ways to wear - you can purchase a belt or a harness for this bag, or use the provided 3-point harness attachment. I just put the bag over one shoulder and under the other arm and swing it to the front. I then put my rucksack on and it keeps it nice and secure.
Top lid - this is padded. Lowepro say you can put your sunglasses in here but I play in Scotland and sunglasses aren't often required. I find it ideal for spare batteries (tucked inside the mesh pocket) and a compass.
Scooped opening - the camera goes in sideways and fit on top of two internal velcro-attached staves. These keep the camera snug and protect it from damage.
Front pocket - this is where I store the dry bag and a chamois to wipe the lens.
Mesh side pocket - I stuff a ThinkTank PocketRocket memory card holder in here and clip it to the camera bag for security.
Zip and clasp closure - You can either zip the bag up or clasp it shut. The zip has big chunky zip-pulls with plastic handles that lets you close the bag with big gloves on (I can even just about do it with mitts on). The clasp saves you having to zip the bag up but I do think it would be better being on a longer strap. I find it slightly fiddly to use.
Size - There's plenty room for a Nikon D4S plus Nikon 17-35mm f2.8 or a 24-70mm f2.8. It also fits a Nikon 70-200m f2.8 lens (with the lens hood reversed). It is a very big bag and I sometimes self-consciously feel a smaller model may be a better choice. This goes away though as soon as I use it.
Optional attachments - you can attach a lens to the outside too with a 'strap and cinch' system for compatible lens cases. I most often use this though for securing a map case. It's ideal for this.
What I'd like to see changed
All Weather Cover - I find this too tight and difficult to put on outdoors. I use a dry bag instead - an Ortlieb 13 litre is ideal - pulling it up over the bottom and clipping it closed at the top. (You can't roll-top it but it forms a fairly weatherproof seal and I can put my camera inside the dry bag if the weather is really bad).
All in all, I think a Toploader Pro is a perfect bag for outdoor and adventure sports photography. It's not bombproof - I fell off a mountain bike whilst wearing it and damaged a lens - but otherwise, I'd highly recommend it.