10/1/2009: The ability counts the number of allies you control as it resolves.
10/1/2009: You target a creature when the ability triggers. You decide whether to have Murasa Pyromancer deal damage to that creature as the ability resolves.
10/1/2009: This ability triggers on this creature entering the battlefield, so it will enter the battlefield with its unmodified power and toughness, and then receive a +1/+1 counter a short time later. It does not enter the battlefield with the +1/+1 counter already on it.
10/1/2009: Unlike other Allies with similar abilities, Kazuul Warlord's ability lets you put +1/+1 counters on all your Ally creatures, not just itself.
10/1/2009: You decide whether to pay {5}{R}{R} as Hellkite Charger's ability resolves.
10/1/2009: If you pay {5}{R}{R}, the new combat phase immediately follows the current combat phase. There is no main phase in between.
10/1/2009: Hellkite Charger's ability may trigger multiple times in the same turn, since its own ability gives it multiple chances to attack. Each time it resolves, you may create an additional combat phase.
10/1/2009: If two Hellkite Chargers attack at the same time, both of their abilities trigger. If you pay {5}{R}{R} for each, two new combat phases will be created. However, all attacking creatures untap as those abilities resolve, not as the combat phases start. Any creature that attacks in the second combat phase will remain tapped during the third combat phase.
10/1/2009: As the reminder text indicates, whether the targeted land has the triggered ability that's been granted to it depends only on whether it has a blaze counter on it, not on whether Obsidian Fireheart is still on the battlefield.
10/1/2009: The ability gained by the land triggers at the beginning of the upkeep of the land's controller, not the beginning of the upkeep of Obsidian Fireheart's controller. The player who controls the land at the time the ability triggers is the one who's dealt damage.
10/1/2009: A land with a blaze counter on it is an illegal target for this ability. You may activate the ability targeting a land, then activate the ability in response to itself targeting the same land. Although the first ability to resolve will put a blaze counter on that land, the second ability to resolve will be countered.
10/1/2009: If all blaze counters on a land are moved to a different land, the triggered ability doesn't follow them. The first land no longer has the ability because it no longer has a blaze counter on it. The second land doesn't have the ability because Obsidian Fireheart didn't target it.
10/1/2009: If a land ends up with more than one blaze counter on it (thanks to Doubling Season or Gilder Bairn, for example), the ability still only causes it to deal 1 damage to its controller each turn.