Too many sustainables cause high fatigue.
Fatigue is a statistic that affects your stamina and mana usage. Many wearable items and some sustained abilities increase the fatigue experienced by a character; typically heavier armors have higher fatigue values than lighter ones. Any talents or spells that are cast by a character have their costs increased by the fatigue percentage.
For example, if Morrigan is wearing only robes and has 0.0% fatigue, her Winter's Grasp spell will cost 20 mana to activate. However, if Morrigan equips some leather armor to the point where she has 10.0% fatigue, the same spell will cost 22 mana to activate.
The maximum fatigue bonus possible is -25%, and any further fatigue-reducing buffs or equipment will not decrease fatigue further.
Fatigue penalty
When there are a lot of abilities being used (cast) at one time or your character does a lot of fighting, a high fatigue will have a substantial effect on your ability capabilities, which requires stamina/mana. Short of taking off your fatigue-laden items, there are a few things you can do to alleviate the effects:
- Lowering the number of sustained abilities at once, which each have a fatigue penalty associated with them, as well as an upkeep (amount of stamina/mana that it "holds" until it is deactivated)
- Cast abilities that will increase stamina regeneration, like a Bard's Song of Valor
- Wear item sets with an enhancement of "-x% fatigue"—in particular, Wade's Superior Dragonskin Armor Set can actually give a character a negative fatigue (of -12.3%), which translates to a decreased cost of your spells and abilities
- This effect can be put to particularly interesting use with mage, and even more so with arcane warriors.
- Enchant your body armor with Endurance Runes available in Dragon Age: Origins - Awakening.
- The problems caused by fatigue can be somewhat mitigated by high willpower and/or gear that adds stamina or mana.
It should be noted, however, that not every character/class/playstyle has issues with fatigue, so before unequipping all that massive armor, make sure it will be worthwhile for your situation. Some PCs prefer the use of passive or sustained talents, so that there is less to manage. Other players prefer active talents might require more micro-management but can be more effective if used carefully. The former PC-type might not care much about fatigue, unless they desire to have multiple sustained talents running at the same time. The latter PC-type should take care with the weight of armor their party members wear as that is often the major source of fatigue.