what to do if you purchased from blizzard (if you didn't purchase from blizzard this will still work for you, but you might want to go to your retailer first, it could be easier) :
1. submit a ticket on the support section of the site, be very clear that you want a refund, not that you are thinking about one, or inquiring about getting one, you are asking for one (don't let there be any doubt), also record the date you requested/confirmed that you wanted a refund and the initial date of purchase. NOTE: if you purchased a physical disc you should include a scan/picture of your game case, receipt (dated within the last 30 days), and Authentication Key.
2. call blizzard support line, whichever one is close to you (details are on the support site under "contact information" or "how to contact us") be polite and clear about it, they should help you, if you get a recorded message telling you they are busy either wait for them to get to you or call again, alternatively you can just hang up and wait because you already have a support ticket.
3. if you got your refund over the phone than mark the support ticket as resolved (or simply add a reply saying you have already got your refund), if you didn't get a refund over the phone add a reply and tell them that you couldn't get one over the phone for whatever reason.
4. be polite, no seriously, be polite and reasonable, the people over at support have to deal with !@#$ all day long, from legitimate issues to abusive !*@!s, most of the time they will be polite to you, and even when they give you cookie cutter responses (which they will) it's only because they want to sort the issue as soon as possible, yelling and being abusive will not get you a refund sooner.
5. leave feedback at the forums to detail why you are leaving, again, remember step 4 and try to give as much detail in your feedback as possible.
6. enjoy whatever game you go on to play.
It really is that simple.
ALTERNATIVELY you CAN get a refund from your retailer, however if it goes against store policy (such as "no refunds on opened items") you will not be able to, simply go to your local store with your copy of diablo 3 and all it's contents (no keeping any of it) and request your refund, some stores may turn you away initially but upon confirmation of blizzards refund policy they may refund you (or not, it really depends on retailer).
a few things to note:
as far as i know "i didn't like it" is not a good enough reason (on it's own), however lag and lack of Australian servers very well may be as you are getting a lesser service than promised because of blizzards choice of server location.
you have 30 days from the purchase date to request a refund, this goes for all copies of the game regardless of whether or not you purchased from Blizzard.
this thread wasn't written with any sort of first hand experience (i haven't and won't be asking for a refund), I am going purely on what I have seen on the forums, blizzard websites and second hand experiences from my friends, so this post may not be entirely accurate, but hey, it's better than nothing right? (rhetorical question, please don't make me feel bad)
now please don't go filling this thread with you're "opinions" on the refund policy, as far as I am concerned the moral ground is shaky at best (not to mention highly situational), but the legal ground is relatively clear and that is really all that matters.
if you feel like leaving your feedback and reasons why you requested a refund on this thread feel free to do so (because it would be nice to have all that in one place wouldn't it?), it may even help keep it near the front page (because while few people use the search function, a few more than a few actually read the top half of the first page)
again, I apologize if any of this information is inaccurate, but if it is tell me and I'll try to update this (better than nothing... I'll keep telling myself that).
refund policy: http://us.battle.net/support/en/article/order-returns-exchanges
submit a support ticket: https://us.battle.net/support/en/ticket/submit
blizzard support contact information: http://sea.battle.net/support/en/article/contact