My own style of photography is a mix of slow studied work (75%) and grab shooting (25%). In the days of film I had several systems and set ups from carry-everything bags that held 2 bodies (one with motor attached), about 6 lenses and three finders, to multiple bag setups for medium and large formats shoots, to quick-access setups that held just one 35mm body and 3 or 4 lenses.
The later is what I have chosen for my new digital system. And since it is zoom lens based (something I never would or could have considered in the old days) I don't foresee ever needing more.
Presently my system consists of a single Canon T2i body with a battery grip attached, 3 lenses (18-55, 55-250, 60 macro). I hope to shortly add a 10-22.
I keep UV filters on each lens largely for protection but also carry circular polarizers in the two sizes needed, a lens cleaning kit, an extra battery, extra memory cards and when traveling a battery charger and a small tripod. The lens shades remain on the lenses, either reversed for storage, or in position when I'm actually on a shoot.
I chose a Canon Deluxe Gadget Bag 10EG to hold my gear, set up for quick access as here:
Note that there is still room for that additional lens and, should I ever desire to add it, a small sensor-driven flash.
The 10E also has well designed straps to hold the tripod and a beautifully designed hidden waist band - useful both for stabilizing the bag for quick and secure access on the move and (just as important!) making it harder to steal by the common (in some areas) 'slash the strap and grab' method. It is highly water resistant, comfortable to carry, and has two closure systems - zippered with flaps and quick connect for when actually shooting. (The later is all metal)