* Swordmages aren't "gishes" or bladesingers. Someone over on EN World made an uncannily accurate prediction about the class, which I can confirm here: A swordmage is a warrior who uses magic to fight better.
* Swordmages use spells of armorning and protection instead of wearing heavy armor. At the most basic level, it's something along the lines of an always-on mage armor spell, renewed each morning. Since they're defenders, they need hit points and AC comparable to fighters, and swordmages get there by using persistent magic effects.I think there are other persistent wards in play too, spells that provide some energy resistance, mental defense, things of that sort.
* Swordmages have lots of room for fun, combat-focused "immediate" spells and "move" spells. For example, my character Geran makes use of a few short-range teleports and transpositions, as well as instant shield-like effects. The movement effects will work great for a defender--what better way to get the troll to stop beating on the wizard than to simply trade places with your unarmored friend?
* Swordmages have room for fun attack powers, too. For example, I have Geran make use of a short-duration, self-only strength spell, as well as another one that wreathes his sword in magical flames. There are a few others I touched on in my novel, but I don't want to give any more away 'cause I don't want to spoil things.