A simple answer is that a human will tend to hit keys in the middle and/or on the 'home row' of the keyboard more and will tend to hit keys near each other together more often (due to double hits). There may also be more 'non-shifted' special characters and number row characters. Numbers and Special characters can capitals are more likely to be grouped (or just appended to) the main 'random' string.
You will therefore get a higher frequency of
asdfghjkltyuivbnm, sequences like
l; etc. and more in the format of
These can be consciously avoided, but this could from its own patten or have a too even mix (nearly the same number of each type).:
This could weaken things somewhat and give the attacker some assistance, but for a long string it's unlikely to be significant. It would also require the attacker knowing it was human generated to they a modified brute force attack
None of my suggestions on frequency is comprehensive and a bit of effort can produce a very random string.
Personally I often use a password generator then change a couple of items or to remove any link to the pseudo-random seed (a bit paranoid really).