[Egil]

That sort of defeats the purpose of diminishing returns on stats, doesn't it?

This. We shouldn't reward people for dumping most of their points into one stat. The diminishing returns should cap at a reasonable amount, but they shouldn't ever increase. Here's an example of something that could work:

-- Strength --
Lvl 1 - 30, 1 point = 5 encumberance
Lvl 31 - 40, 1 point = 4 encumberance
Lvl 41 - 50, 1 point = 3 encumberance
Lvl 51 onward, 1 point = 2 encumberance

-- Skill
Lvl 1 - 30, 1 point = 1 critical
Lvl 31 - 50, 2 points = 1 critical
Lvl 51 - onward 3 point = 1 critical

These are raw numbers that would require further balancing, but it is definitely worthwhile for some builds to invest 80 in.