Ignis the Furnace Master has been tweaked by Blizzard to lower his DPS in Heroic Ulduar 25-man.  Some guilds are reporting their main tank is getting one-shot by Ignis within 20 seconds into the encounter.  Blizzard will compensate by increasing his hit rating.

Blizzard Quote:
We’ve made some changes to Ignis’ melee attack on Heroic. We reduced his overall melee DPS by about 15% and he swings faster which will make his individual hits less devastating.  In normal mode we increased the cooldown of Activate Construct from 30 to 40 seconds.
We also decreased the damage of the Scorch ground effect by 20% on both difficulties. On a side note, Ignis has always had the flag that prevents parries from making his next melee swing happening faster. We’re still working on the people being melee’d in the slag pot issue, this one is proving tricksy.