Lanefan
Victoria Rules
And the obvious one:I am not sure if there is a hard and fast rule, but I think that generally, if an acronym easily resolves (due to vowels), and is more than one syllable, then it will tend to get sounded out as a word. If it's one syllable (especially if it's three letters or less) then it will get said as the letters.
Hence-
UN is Yoo Enn.
But UNESCO is Yoo-Ness-ko.
UPS is Yoo Pee Ess (not, um, ups. Although that would be funny!).
CONCACAF (a soccer thing) is Kon-Ka-Kaf.
How do you prononuce ENWorld?
For me it's "Enn-world" but some would say it as "Ee-enn-world".