I remembered reading some stuff on how steep is too steep to ride etc as well as the impact on fatigue etc but couldn't find it until now. NOTE that a %percentage gradient is not a direct equal for °degrees, eg. 25° doesn't equal 25% (hope I got that correct).
http://www.wired.com/wiredscience/2013/03/whats-the-steepest-gradient-for-a-road-bike/
http://theclimbingcyclist.com/gradients-and-cycling-an-introduction/
http://theclimbingcyclist.com/gradients-and-cycling-how-much-harder-are-steeper-climbs/
Whilst these articles are from the road riding scene we can apply to mtb, but mtb I reckon has increased levels of difficulty due to the surface trails not being consistent, loose rocks, leaf litter etc which means we have to fight or position the bike and us on the bike to maintain traction whilst keeping the power down and bike ascending. On the roadie some of the uber steep stuff can be done in the dry but then needs you to remain seated in the wet to keep traction. Whether mtb or road you need the power, technique, fitness and gearing to help you get up some of this stuff. Many times not knowing the exact numbers before you climb or are in the middle of climbing helps by not doing your head in subconsciously by making you think this is hard, but after you make it sometimes feels like yeah I can do it, because you didn't have any excuse, you just had to give it a crack.
But also saw an article which is quite simplistic but true, that had the title "When is a climb too steep,....when the Pro's have to walk"
http://road.cc/content/blog/78218-when-climb-too-steep-when-pros-have-walk
