Hi,
you are actually quite close. The first thing is that you don't actually need to join the strings. I made the labelling a bit nicer.
BarChart[#[[All, 2]], ChartLabels -> #[[All, 1]]] &@Reverse@SortBy[Tally[Flatten[Characters /@ ToLowerCase[WordList[]]]], Last]
I have also sorted the letters with respect to their frequency. The code I suggest is not the shortest one but should be easy to understand.
Cheers,
M.
