In vanilla, warlocks seemed to have a very specific niche - wielders of demonic magic and dominion over demon slaves.
In cataclysm, I don't see a niche for warlocks. Maybe I am not digging enough into the class differences and, if so, I am happy to be corrected.
As a pure dps class, that is our bread and butter, and honestly i have no complaints...warlocks may not be the leet dps they were with SB spam in BC, but we are certainly competitive and perform well from a dps standpoint.
CC - used to be a big deal, and used to be considerably limited to mages, rogues, hunters and locks.
CC is now the business of locks, mages, hunters, warriors, priests, paladins, shamans, rogues, druids, dks...it seems there is a CC for every class.
Debuffs - warlocks were great to have in vanilla and bc raids because of debuffs they provided - particularly resistance debuffs that were consolidated into curse of elements for wrath.
Not only have the quantity of debuffs increased, making each one theoretically less important, but debuffs have been given to more classes in an attempt to keep locks (or any class) from feeling mandatory - but at the same time marginalizes the class.
Pets - this was initially what drew me to the warlock...it's not just a lone soldier...he has a buddy! Now there are a potential 4 classes with a perma-pet. I still don't consider mages and dks pets to be on par with lock or hunter pets. While they are permanent the mage or dk have only one pet to choose from with limited utility. Now, the hunter is another story. I have always envied the number of pets they have to choose from, but felt that this was balanced by the inability to readily call on any one of them. The hunter chose a pet and was stuck with it (until he got to a stable master) and this seemed an even trade. In cata hunters can have 5 pets available at all times (out of combat) which is more than non-demo warlocks...huh? Well if they're going to do that can we at least talent our little pets too?
Raid utility - the warlock is a bit limited here, which is understandable given that we are a high dps class. At one time healthstones and soulstones were a necessity...I don't see this happening again and I'm ok with that, but the warlock offers little to a raid outside of his individual damage. We don't have buffs (we have one buff that we can use to buff one person...doesnt count lol), and soulstones are stupid! - yes they can be useful...if someone dies, and you are able to accurately predict who will die, and they are able to die in a place where they can survive being resurrected with little health and mana...lets face it...this is a stupid ability that can be good...but typically is not.
Draining/Self Healing - this, IMO, was the biggest difference between warlocks and mages, as well as all other dps classes. The warlock is a drainer when he wants to be...taking your mana as his own, taking your hp as his own. This had PvP use obviously but was also really helpful in raids where mana conservation was important - if your healers were running low you would DL/DC/bandage to help out. For the sake of "PvP equality," everyone and his cousin can heal themselves in some way - with the rogues being the last to get this nifty ability. In addition a lot of specs/classes have passive self-healing abilities or procs...i still dont understand the logic of a warrior healing himself btw.
I guess my dilemma is that I fear we will not offer anything special in cataclysm and it would be much more beneficial to pick a hybrid class when working towards progression - i don't want to retire my lock - i just want it to be a bit more useful.
There is a fair chance I am mistaken or have overlooked some logical fallacy...before you respond keep in mind, it's just a game.