I'd also be interested in hearing about the differences between the two domains of the warpriest if Fiery_Dragon is still answering questions.
As has been mentioned before, Storm is more attacking powers and Sun more protective powers. Here's some good fluff from the Storm section: "Kord is the god of storms. A mighty warrior who revels in strong drink, he is moody and prone to anger, laughter and gloom in equal measure. He expects his clerics to match his thirst for battle. Although Kord is a force for destruction, he has no stomach for bullies. Wanton devastation or attacks on the weak and defenseless draw his fury."
Sun fluff: "Pelor is the god of the sun. He is a kindly diety who expects his priests to serve as his emissaries across the world. As a cleric of Pelor you tend to the sick and wounded wherever you find them." etc. etc.
Your choice of Domain also has a mechanical effect on what powers you choose. For example, when a a Storm cleric uses Healing Word class feature on an ally, the target gains +2 to their next damage roll. The storm at-will powers are: blessing of wrath (weapon vs. AC, one ally in 5 squares gains power bonus to damage roll) and Storm Hammer (weapon vs. fort, when charging can use this in place an MBA).
Sun Domain: When use Healing Word on an ally, you or the target gets back 2,4,6 (by tier) hit points in addition to the Healing Word effect. You also grant all allies in 5 squares a +2 bonus to death saving throws at all times. At-wills: Blessing of Battle (weapon vs. AC, you or ally in 5 gains resistance to all damage equal to your CON mod til end of your next turn) and Brand of the Sun (weapon vs. AC and you or ally in 5 can make a saving throw.)
3rd level encounter examples:
Storm: Hammering Wind. Weapon v AC (push the target one square and knock it prone)
Sun: Resurgent Sun: weapon v. AC (2W) + you or ally in 5 can spend healing surge.
So there's a marriage between the fluff and crunch differences between the two domains. So far the powers I've seen have been pretty sweet overall, and everything is keyed off wisdom for simplicity. Everything I've read leads me to believe that Warpriests from either domain are going to make awesome martial clerics, as every attack not only hits hard, but simultaneously buffs or heals you or your party. I would want one in my group.