I like a belt type bag that also has a shoulder sling.
1. Relieves the back to carry a day pack
2. Distributes the weight from waist down (less shoulder/back ache)
3. Can choose to shoulder sling if necessary (eg. easier access or in crowds)
4. More mobile/agile w/o the bag swaying here and there in the case of shoulder bags.
Downside :
1. Typically such bags are smaller, so can't fit in very long lenses (eg. 70-200/2.8)
I manage to fit 3 lenses, 1 flash, filters though
Not really, Think Tank Speed Racer fits you criteria