Guardians specialize in protective and defensive magic. A deep sense of loyalty to their allies fuels their passion and power. As a soldier profession, guardians wear heavy armor.The guardian feels like a tank, with the option to deal some damage.
You can choose to go for survivability and stand as the last man or you can let go of that defense and deal some damage while still taking alot of damage.
The guardian, compared to the warrior, has a more magical/paladin feeling, meditations, consecrations, summoning weapons to help you and your allies.
Another note, the guardian has some nice mobility options, with a sword you get a teleport (to your enemy) and you can take a skill that teleports you to your foe and burns everyone around you. Those 2 combined let you jump from target to target and set them on fire while your at it. (The teleport with AOE fire has a decent cooldown, but not to long, while the sword teleport has a short cooldown and blinds your foes, use it alot to avoid damage).
While I also like the Greatsword (sping, might buff,..) I prefer a more defensive build with sword/shield.
The Guardian, thus far, feels like the heavy-armored class that it portrays. A typical weapon loadout of Mace/Shield + Sword/Focus gives you the following abilities: an AOE ground effect that heals you and burns foes, a block that damages foes, a wave of protection (take less damage) for yourself and all allies that also damages foes, a protective dome that blocks projectiles and pushes all foes outward, a blind, a block (for projectiles) that also damages foes, an AOE blind that also heals all allies, and an ability that blocks 3 attacks.
This isn't even taking into account the passive/active effects of the Guardian's virtues or traits, which add more to survivability.
As with all professions, you can use the Guardian offensively or defensively, but this profession lends very well to defense. I tend to take very little damage when switching between these two weapon sets, and have a lot of fun.