I guess I'm a minimalist. My basic road seatbag includes tube, patches, tire lever, and a Park multi-tool (the small flat one with really basic hex sizes and a flat screwdriver). I have CO2 and an inflator in there too, but I really only use CO2 when racing or on a fast group ride. I carry a baggie with my insurance card (doubles as ID) and a few dollars in my jersey, along with a small pump in my jersey pocket.

My mountain bike set-up is the same, but I also have a chaintool and a slightly beefier multi-tool. I also carry a whistle (from my backpacking days ) and a compass. If I am riding in unfamiliar terrain I'll include a map.

When I ride alone I carry my cell-phone.

I have a Camelbak, but I use it more for hiking than riding. I too hate the feel of a "backpack" when I am on the bike. If I did loooong all-day back-country mtb rides I'm sure I'd use it . Right now my mtb rides tend to be only 2-3 hours so it seems like overkill.