Casual rundown on what i've found to be the case: (Remembering that how good the player is is much more important than class, and to some extent gear)
Mages: Ice are my least favourite match up. Ice blocking your bubble, spellstealing your BoFreedom. I can't beat a really well played ice mage in a duel, and will often dread my rogue partner dying before me in a match against one. Fire mages can be tricky, but have less CDs and are a bit squishier.
Priests: Disc priests are another nightmare. Even if you are quick enough to interrupt Mass dispell, it's a spammable ability. And failing to interrupt the big heals means a loss as well. Try hitting Avenging wrath before it can be dispelled for huge damage. Spriests are much softer targets, your burst should annihilate them before mana burn can mess you up too much.
Warlocks: Another tough match, but the easier of those i've mentioned. When howl of terror hits, wait for the follow-up fear before trinketing (or bubbling). Comes down to gear, and how quick you are with interrupts. Afflic/Destro locks are much easier kills than monstrously durable sl/sl locks.
Warriors: A free kill. You can spam repentence/HoJ to stop them charging you, use freedom to stop hamstring > intercept and bubble/BoP disarm. Smart warriors will have a macro to swap out a sword and board while you are invulnerable.
Paladins: Mirror match! Ret all comes down to saving interrupts for enemy heals, and NOT bubbling first. Bandage/drink if they use Divine shield defensively, try your best to stay out of melee range if they use it offensively. Holy paladins are unkillable. (You *will* run out of mana/interrupts. Holy shock will be where your life keps going) Prot don't exist in a pvp context for obvious reasons.
Shamans: Elemental/Enhancement are fairly simple to beat. Judgement of Justice always to stop them escaping in GW form to heal up. Resto shamans are like fighting holy paladins.
Hunters: Bubble as much of The Skill Within as possible while piling on damage. Freedom keeps you in range, hammer of justice and repentence if they still aren't dead when those CDs are used. I find hunters a fun easy fight (because if anyone ganked me while leveling, it was usually a hunter).
Druids: Judgement of justice again to prevent fleeing. Blessing of freedom when you see an entangling roots coming. Stun them in caster form for great justi.....damage. Use AW rather than bubble to burst them down before HoTs can tick for too much. Feral will get an opener, immediately trinket and stun. Bubble when they go bear and Bash/Feral charge. Boomkin are deceptively tough, but none of their spells are an inherent counter to retadins.
Rogues: A rogue with all their CDs available will likely just beat you, like they will many classes (you can be satisfied that for the next 5 mins they are a walking honor kill). They can vanish twice, blind, gouge. Even if you seem to be killing them, they can likely get away. Trinket the kidney shot, and save hammer of justice for evasion (when they turn "see through", can't dodge while stunned). If they gouge/blind and try to break away to restealth, bubble, repentence, do whatever to stop them disappearing. Consecrate will do nothing to stop a rogue opener, but can be dropped during the fight (rank 1) to hopefully catch a vanish.
This is all incredibly generalised, just an overview of which classes are likely to be harder than others.
Edited, Sep 2nd 2008 11:45pm by ArtemisEnteri