See notes about uncovered pitches passim.
There's one other factor that I've never seen anyone mention in any 'then and now' debate - and that's protective clothing.
Batsmen up until the 1970's wore CAPS (

) when batting - and I've often wondered whether being aware of the possibility that the next ball that you face might be the last you ever would, ever swayed a batsman's judgement.
How many times have you seen modern batsmen get hit on the helmet 2 or 3 times in an innings, yet go on to score heavily?