1800 / t3 mogs are bis mogs yo (darkmoon faire replica sets are worth looking at too)
t1/2/2.5 look cool enough tho
some are ugly but some look nice
there are actually some nice green boe sets you can just buy off the AH, I know leather has some cool ones, and if you don't have a plate wearer to farm out the tier sets its like one of the only ways to get a cool plate mog that isn't trial of style
but to regurgitate what everyone else has already said: level 1s and up can mog any and all gear pieces up to level 60