the thing about free verse and also about rhyme/meter is that either way you have to truly think about which words you should use to best express your feelings/ideas/observations.
What can happen is some people get lazy with free verse, but equally so, some people get lazy with rhyme/meter too. Generally it is the laziness rather than the method itself that causes the failure.
Also, it should be realised that free verse is not actually without rhyme or meter, it is just that the rhyme and meter are varied/natural. free verse lends itself to change of tempo in a way that formal verse never can. This is why good free verse can tug the strings of emotion/thought in extra ways that formal verse cannot.
It might be likened to the difference between a private conversation and an oration.
It doesn't make one good or one bad - there are examples enough of brilliant and terrible of both.
Back to iambic pentameter, what hasn't been mentioned yet is to explain that the rhythm is 'unstressed-stressed' - and, what I THINK I recollect is that this is what 'iambic' actually stands for. the pentameter being x5 making
unstressed-stressed unstressed-stressed unstressed-stressed unstressed-stressed unstressed-stressed
more commonly known as ta-dum ta-dum ta-dum ta-dum ta-dum !!!