Curse of Weakness easily makes my list of the top five most useless warlock spells, (I actually made that list once.) The pittance of attack power (478 at level 80) that this spell removes from the target is so insignificant that it's not even really viable as a PVP talent. And while it can be improved by talents, it can't be improved nearly enough to avoid much more useful talent points, such as the improvements to drain soul, life tap, and drain life. In complete honesty, the only time I can think of that I might use this spell, is in-between levels six and eight, when it's the only curse that I have. (Or, I suppose, if you're in a raid with an inordinate amount of warlocks--but even then Curse of Agony or Doom is probably a better choice.)
Curse of Recklessness is one of the most under-rated curses in the game. Not so much for the armor reduction, which is really just incidental, but rather for the fact that it causes mobs not to flee. This can be extremely handy in instances filled with humanoids, where a mob with low health is often likely to run off and pull a second group of enemies, causing the wholesale slaughter of a warlock and his or her compatriots. Back in the days, before all these whippersnappers started playing the game, some warlocks used to practice a tactic called 'yo-yoing,' where they would fear a mob, CoR it, then throw another curse on it and fear it again--effectively causing the mob to run away from, and towards them over and over again. Unfortunately as of patch 3.0.2 CoR no longer causes creatures to ignore fear effects, but it still prevents them from fleeing.
Curse of Tongues I like this curse. I actually like it a whole lot. I just don't know what to do with it. I imagine it'd be rather handy in PVP, but I don't really PVP all that much. Never was able to get into it. As such it spends a lot of time next to curse of weakness on my action bar, gathering dust. However, it DOES make the stuff you say sound funny. And that's just cool.
Curse of Exhaustion Much like Curse of Tongues, I love this curse, yet I never get to use it. Though, much unlike its counterpart, the reason I never get to use it is because I never get to *have* it. Curse of Exhaustion is great for kiting, which is something warlocks could certainly use help with. If I was to create some kind of soloing spec, or a PVP spec even, Curse of Exhaustion would be a must. However, in the instancing/raiding specs that I use to optimize my performance in what I consider the best part of the game, the two talent points required to get this little gem are much better spent elsewhere.
Curse of Elements is my curse of choice. Not only does it give a huge buff to all of my damage, but it gives buffs to the damage of my buddies who play magi as well! What's more, now that 3.0.8 has gone live and endgame BM hunters start using arcane shot, it'll help them too! By far this is the best curse to use for warlocks in an instance, unless there is some other warlock there casting it (since the effect does not stack,) or some other class casting a debuff that CoE doesn't stack with. (Boomkin and Death Knights do this.)
Curse of Agony is, according to every single source (reliable and unreliable) the curse you should be using in Wrath of the Lich King. You can distinguish (roughly) between the reliable and unreliable sources, because the reliable ones will add "unless nobody is already casting Curse of Elements or something that supersedes it" under their breath. Honestly, I'm still having a bit of a hard time with this. I believe I may have mentioned in the past that when I was being raised, a warlock could get quite a talking to for using Curse of Agony in a raid. I remember one raid leader once saying "Could all the warlocks putting Curse of Agony on the boss try doing something USEFUL instead?" As such, I've been conditioned against using it, A Clockwork Orange style. That said, I've begrudgingly accepted that it's useful these days, and as soon as I find a boomkin to hang out with, I'll start using it.
Curse of Doom was once called "Curse of Takes-forever-to-proc" by my guild's main tank. Back in Burning Crusade (damn, I say that alot,) this was the spell you were supposed to use if somebody else had Curse of Elements up. And at level 80, if you don't have points in Improved Curse of Agony, the 11k damage of this spell still ends up being slightly superior to CoA according to my damage tests. Though whether there's actually time for it to proc depends on whether or the mob will stay alive forever I suppose. That said, if you're using damaging curses rather than CoE, you should probably have points in Improved Curse of Agony.
Really, the layout of curses right now is pretty dismal. You'd think that a core class mechanic would be less cast aside than curses have been. Healers get more and better ways of healing, DPS gets more and better ways of DPSing. Why doesn't the debuffing class get more and better ways of debuffing? This is something I plan to address in more detail in a future post.