The Iroquois are a highly underrated civ, IMO (or were; my Conquests copy hasn't arrived yet. =/).
As others have stated, expansionist can be a very powerful trait under the right conditions, and religious is also very useful.
In addition to a solid trait combo, they have an excellent UU; it has the attack power of the swordsman and the mobility and retreat abilities of the horseman. Horsemen cannot run from them, and swordsmen are easily outmanuvered (in most circumstances). Because of their speed, reinforcements arrive very quickly.
The only bad thing is that, if you want to get the most out of your mounted warriors, you may have to initiate an early GA. That isn't too bad, though; the earlier you establish a mighty empire, the more points you'll get in the end, and the MW is one of the best early UUs for that purpose.
I usually play from regent to emperor. With the bonus units the AI has early on in deity, early conquests are harder, and the mounted warrior is less effective.