1st up, I don't buy gems. I'll watch my weight in ads, but paying for gaming to me is like the idea of paying for sex, it takes a lot of the fun out of it, and just feels a bit wrong.
I do most of my HoD playing in Australia. Gems here are VERY tough to come by. I get offered 0 to 5 ads to watch most days, and every now and then I'll get some kind of miracle jackpot of around 10 (averaging 3 to 4). I try the free gem button at least 20 times in a day, at various times of day (I play quite a bit - 314 stars).
I recently spent a few weeks in the US to visit family, and booted up HoD to find you guys get about 30 free gem opportunities a day!!! Jack in the Box ads alone probably bought me 3 heroes while I was stateside. It was the same at the tapjoy website... big opportunities to earn gems up the yang compared to the trickle we get in Australia, and probably most of the world.
With the amount of heroes, weapons, 100% chance upgrades etc that are gem-dependent, it really does make the game ridiculously easier in the US. All I seem to do here is wait for events so I can buy gems with my points to progress in the game, but in the US I'd get months worth of gems in literally days.
That's not meant as a cry or rant... just an observation that US players get their hand held an incredible amount by glu/tapjoy compared to the rest of us suckers.