Fit, above all else. I have a hard time finding Merrells that don't constrict my toes (especially downhill) although they make many good shoes. Keens are by contrast quite roomy and their ample toe armor is welcome on rocky trails and XC. They're relatively new to hiking boots/trail shoes, and seem heavier than the direct competition, but wear well and are certainly worth a look. Have also had good lock with Asolo boots, but less luck with their lighter weight shoes and sneakers.

My "ideal" trail footwear would be lightweight, mid-height (over ankle), well ventilated with tough Vibram (or equivalent) soles. They should be torsionally stiff (resist bending along the sole axis) and have a rugged toe cap. Because nearly all mid-height trail shoes are waterproof or wpb, I can't really find this shoe in the current market. As a result in summer I switch from mid to low tops, which means whacking my ankles and either wearing gaiters or getting shoes filled with trail debris.

I also frequently have delaminating top caps and torn lugs, as most trail "runners" prove a poor match for Sierra trails. Returned a pair that were falling apart after a fifty-miler last summer. OTOH nicely groomed forest trails don't take much of a toll on footwear, and you could probably wear bedroom slippers.

When trying them, make sure to test them on a downhill slope to check for toe contact. The toenails you save might be your own.

Good luck!