I think that shields should be more interesting than picking the color you want to glow when you press your shield button. Does anyone else agree? And no the 25% elemental resistance is not enough to sway one's decision about which shield to pick, the main factor is if you use a +elemental power enchant and the shield matches that element.
What I mean is, make each shield more different. In order to discuss changing the shields, however, we'll first have to establish their current form. They currently last 12 seconds or until they break. They drain mana while active. There is nothing stopping you from just turning it back on after it breaks. With perfect timing you're effectively invincible against almost everything, while not using very much mana. Without perfect timing, you use mana to attempt to block hits, and will sometimes waste significant amounts of mana to do this.
By modifying, adding to, or removing from these traits we can make each shield different quite easily. I can throw out a few ideas real quick but I would like to hear others' input mostly.
Fire shield: Deals damage to enemies on contact.
Water shield: Less mana usage but you take 10% of the damage through the shield? Or maybe regenerates shield health over time?
Air shield: Blows away enemy projectiles? Shorter duration.
Earth shield: Reduced movement speed while in effect but more durable.
And, since why not, even if they aren't in the game currently...
Light shield: Maybe rename to force field? Knocks enemies back on contact.
Entropic shield: Uhh... Doesn't block (much?) damage but increases your own power depending on how much damage the shield has taken. Doesn't break but still has a time limit.
Besides all this, have shield damage decay over time instead of instantly. Being able to bring up the shield at full health immediately after it breaks is not fair. Also make it have a short cooldown before you can bring it back up after it breaks fully.