Think outside of sets. Anything outfit that has a set bonus is going to be popular, no matter the level. The Scarlet Monastery set is a particularly good example of this. Try to find a way to use individual pieces from various armors in a complementary way. If you still like the gear to match more closely, look to some of the rarer sets like the Blackrock Depths gladiator set, or sets that don’t have a set bonus, such as the Chief Brigadier’s armor. Be prepared to spend a lot of money and/or time if you decide to pick a set that is a world drop BoE, though.
Certain graphics are used for multiple armor types in rare circumstances. The wolfskin hat/mask graphic is almost exclusively used for leather and mail…except for a certain quest in Terrokar Forest that gives cloth and plate versions as well. Sites like Wowhead have a feature that lets you see all the pieces of gear that share the same model and are quite helpful in situations like this.
If you are looking for a unique weapon to wield, take a look at the drops from heroic Burning Crusade dungeons, especially the ones that were rarely done. There are some absolutely unique and cool weapons that most people never saw, even in Burning Crusade, just because the heroic they came from was too difficult or time consuming for badge farming.
Hold onto everything. Transmogrification and void storage came together in the same World of WarCraft patch for a reason. Also, even if you can’t transmog a particular piece of gear now, you might want to hold onto it anyways. Blizzard has already stated they plan to relax more of the restrictions on transmogrification in the future because the overwhelmingly positive response they have had to the feature. If you like a particular model but not the color, methods to change color might be in the future as well.
Transmogrification can even give you a tactical advantage! In organized PvP such as arenas and battlegrounds, enemies have few reliable methods to ascertain your capabilities. Your enemies cannot see your stats, so they can only go off of what they see. With transmogrification, you can control what they see. Now, players will not just take what they see at face-value if it doesn’t make sense. If you go into a level 90 arena match wearing what appears to be level 15 greens, everyone will just correctly assume you transmogged your gear and not give it a second a thought. The key is to make the deception palatable. Dress yourself in an assortment of crafted blue PvP gear and questing greens. Hide your gladiator epics behind whatever is one of the more current PvE tiers. In world PvP, you can present yourself as a weak target by hiding your glorious conquest gear under greens and provide any would-be gankers with a nasty surprise.
Speaking of World of WarCraft PvP, if you happen to have a title from the vanilla PvP system, you have the unique privilege of being able to transmogrify your armor and weapons to look like whichever pieces of the old vanilla PvP gear your title entitled you to. Champions and above can wear the entire blue quality set and warlords and above can wear the epic version. Because of some oddities with the data for those pieces of gear, you will have to go purchase the replica versions from a vendor in Area 52 to transmog, even if you still have your original pieces.
Never underestimate the value of looking cool, especially in a video game. If you make a truly unique outfit, expect compliments and plenty of whispers of “how did you get that?!” Most importantly, even when your guild wipes for the tenth time in a row on the last phase of heroic Madness of Deathwing, you can at least take consolation in the fact that nobody else looked as good as you while doing it.