Remember, these are straightforward answers to straightforward questions. Things can get complicated and almost everything has an exception.
Q: I got a ______ weapon. What should I reroll?
A: There is now a damage calculator for this. Remember that IAS basically only affects hydra and Firedawn build (high group GR). IAS does not increase your perma Firebird DoT, your blizzard, your meteor or your Black Hole damage.
Q: I just returned/hit lvl 70/have no idea what I am doing. What should I get/use for a build?
A: Do you have Firebird?
You can switch stuff at random (armors and passive especially) pre FB.
Q: %Fire or Intelligence on Amulet?
A: Almost always %Fire - There are rare exceptions on ancients with very high intelligence if you have very high %fire. Fire/CC/CHD/Socket is BiS.
Q: I have X and X, which should I use? I need to reroll X item, what will give me more dmg?
A: Use dolynick's spreadsheet
Q: Which legendary gems should I use?
A: The general consensus is Toxin, Pain Enhancer and Zeis for T6 and Tox, Zeis and Bane of the Trapped for Greater Rifts
Q: I don;t know what stats I need when I am rerolling/equipping. What should I do?
Q: I just found a furnace and I don't want to mess it up. How should I reroll it?
The equations apply to an ancient Furnace as well.
Q: I can't move fast enough in group t6? Why is everyone faster than me?
A: http://us.battle.net/d3/en/item/warzechian-armguards and Ice Armor/Frozen Storm with Teleport Wormhole and Illusionist passive. Aether Walker, A Season 1 item will be available in non-season soon.
Q: How should I allocate my Paragon Points?
A: MS>INT. You can throw some into vit or Max AP if you feel you need it.
CHD=CC (its complicated, simple rule: 2hand weapon =CC, 1h = CHD) >CDR >IAS.
If you are running hydra, use you paragon points on ias to get to the next breakpoint.
Resist All > armor > life > Regen
RCR > AD > LoH > Gold find
Q: What are these breakpoint thingies?
Q: What other elements can I use besides fire?
A: Anything other than fire is going to be inefficient. That being said, you can technically use both Cold and Lightning. Look up some builds on either or search the forum.
Q: I'm having trouble with Greater Rift Level X, what should I do?
Q: I see all sorts of abbreviations what do they mean?
This will be consolidated at some point.
Q: I haven't gotten X? How rare is X? How many Blood Shards for X?
Q: What weapon should I use?
A:) It obviously depends on the rolls, but assuming everything is perfect and you are running T6: Devastator <Maximus<Sun Keeper <Anc Devastator <Furnace <Anc Maximus <Anc Sun Keeper <Anc Furnace.
Serpent Sparker and Wand of Woh are alternatives that work also, but it is a different build. Serpent Sparker is roughly between Maximus and Sun Keeper at a rough estimation (total wild guess).
Wand of Woh is a melee build and uses CDR, so it's entirely different, but fun and effective.
There is currently a debate about Ancient weapons and white mobs vs. elite mobs in grifts. Let me know if I am off please, I did this off one the top of my head.
Q: X dropped. Should I keep it? How do I reroll it?
Q: What the hell is this Firedawn build thing I keep seeing?
A: It is used for high Greater Rifts and requires full crowd control (horrify Witch Doctor). You will die if touched. Note that Blazing has ben replaced by Mammoth as of 2.1.2
Q: I have this Ramaladni's Gift and X weapon. Should I gift it?
A: Keep at least one gift in your stash in case you get an Ancient Furnace, Serpent Sparker, Maximus, Sunkeeper or Wand of Woh. Those are all giftable. A crafted Ancient Devastator is also giftable. You can consider gifting non-ancient Serpent, Furnace or Woh. A Smoldering Core for trials is also doable, but not recommended. Anything else is technically a waste, but, hey, it's a game and you may find a lot of them.