As for Shadow Priests we know they have scaling issues in the upper tier raids but there are two raiding shadow Priests do excellent DPS in addition to their valued mana return.
   Shadow Priests respec because Holy Priests burn out so fast (almost without exception), and their friends convince them to do it, not because Shadow isn't an absolutely stellar spec. It's one of the best "DPS and PvP"-type specs, and it excels in both, it really does (as long as you're smart and don't think "Shadow means stay in Shadowform until we're on 20% health then drop form just in time for the Execute."
We're really impressed with good PvP Shadow Priests, they scared the crap. It admitted that bad ones are total fodder, but there are few classes for whom that are not true.
Elemental Shamans do really good DPS in raid and 5-man situations, and are also hilarious, because lightning so much cooler than Fire or Ice, also buffs. Also being able to nuker and heal and being pretty tough and able to chase people in RvR pretty much = win against all but the smarter players, and even those if you play smart too.
Destruction Warlocks right now are absolutely monsters in raiding situations and 5-mans. In PvP, if they have good gear, they're deeply horrific because they're capable of inflicting.
We played a raiding shadow Priest for over a year and only recently went holy. They're fine (it was on a PvP server, so nearly a necessity) and are well regarded in raids. VE with VT can do some pretty impressive things. Melting faces is a lot of fun. With that said, we were being asked more and more to heal and we like being really good at what we do. If we wanted to do straight DPS we would have rolled a DPS class, so that's why we went holy.
As a WoW beta player who started with a Warlock and still play that same one today, we have a low opinion of destruction Warlocks. Sure, they're viable, but we can't get past the wannabe mage mentality. Affliction and demonology are so good that choosing to go destruction seems like such a waste and doesn't play to the Warlock strengths.
Thanks to lack of the survivability, PvP Shadow and PvP Destruction don't show up much in Arena, in particular to melee. Elemental Shamans have a reasonably good presence.
Due to haste and crit not having much of an effect on damage, PvE Shadow scales very badly with gear (a problem Affliction Warlocks also share - DoTs don't crit and aren't speeded up by haste rating, 150% crit damage nukes don't hit very hard), to the point where a T6-geared Shadow Priest is doing 1400 DPS to his rogue and warlock guides' 2000+ (or more) DPS. However, it is highly competitive at the T4/entry T5 level where it scales well before the raw spellpower value on gear plateaus in favor of mixed power/crit/haste. Shadow Priests attend high-end raids for their mana battery, Warlock buffer and passive raid healer roles, not their damage.
Destruction Warlocks running the 0/21/40 Demonic Sacrifice spec top DPS meters in BT and Sunwell due to talent stacking multiplication and extremely good scaling with all three caster-gear stats. Mages are jealous of them, not the other way around. Unfortunately, sacrificing your Succubus and spamming Shadow Bolt until the boss dies is terribly, terribly boring. You don't even cast dots because the GCD time is better spent casting more Shadow Bolts.