I must be stupid or something. I've been writing libretti and poetry for years and I've never been able to fathom how the great opera librettists of the eighteenth century working with composers like Handel and Mozart could churn out yards of the stuff at the drop of a tricorn hat. It didn't occur to me that there was something specific about the Italian language that made this possible. I found this on several sites (original on Wikipaedia, I think):
"In English, highly repetitive rhyme schemes are unusual. English has more vowel sounds than Italian, for example, meaning that such a scheme would be far more restrictive for an English writer than an Italian one - there are fewer suitable words to match a given pattern. Even such schemes as the terza rima verse form with a rhyme scheme: ("aba bcb cdc ded..."), used by Dante Alighieri to write The Divine Comedy, have been considered too difficult for English."
There you go, the Italian librettists had it easy.