In the early levels I agree with you completely. They are small, bland, and don't quite compare to the cloak and sash of the Hunter and Titan classes.
At higher levels, however I believe the reverse us true. The Titan's sash gets a little longer, more colorful, and a few even partially extend around to their back. The Hunter's cloak can only grow so long (and it does) and also gets colorful. But the Warlock's armband becomes a work of art. Not only is it the only of the three to become illuminated, the designs are amazing! They (no pun) outshine their other guardian classes.
So if Warlock is your favored class, don't fret. You're just paying your dues. At level 20, those other guardians will envy YOU.