BBs need facing
Wheelbuilding is difficult
29ers inherently suit taller riders better
CK headsets are good value for money (at usual retail prices)
Steel frames are all springy, aluminium ones all stiff and dead.
Carbon is crap for mtbs because of lack of impact resistance
Wavy/bent/skinny seat/chainstays make the back end of the bike comfortable
8/9/10 speed is weaker and wears significantly faster than the previous iteration and clogs with mud more.
Pretty much any material science stuff that the mags print (which I note ST mag rarely does – thumbs up!)
You NEED x inches of travel for a particular type of riding or conversely that just because one of the STW-heroes has cleaned a 2 mile vertical drop on a cx bike with a spike for a saddle, skinny tyres at 1000psi and no brakes that everyone else can do so and more importantly will enjoy it.