I like warlocks a ton but I know some of their functionalities aren't what you'd think. They're essentially a sustained DPS class (which makes them AWESOME at killing bosses, especially at high levels) and crowd controller. When you use the succubus you can literally take up to 3 mobs out of a fight with seduction (on humanoids, the most common type of mob), banish (if demon/elemental, and there are a lot of these at higher levels) and fear (of course not recommended in an instance, but I actually use it pretty well outdoors). I'm fine with curse of agony being the way it is because it makes the class much more viable in pvp - since it's a curse, only mages and druids can dispel it, so it makes the warlock a lot more effective against everyone but those 2 classes (and for those 2 you get the magic DOT from immolate and corruption). I also like the way the damage works on it because it makes managing aggro a lot easier - early on in the fight, agony is doing low damage so the tank has a better chance of getting more hate, while the curse builds up at the end. It's also ridiculously strong. They've reworked this spell three times - at first it did the same damage it does now, but over like 1.5 minutes, so it was useless for soloing or instances, except at high levels on some of the bosses. Then they changed it so it did less damage, but over 30 seconds, and it was fixed damage every 3 seconds like all other DOTs are. This was ok by me, actually, but then they went back to the old damage numbers, but then made it work on this "build up" damage system. Just to compare the higher ranks of the spell... the highest does well over 1000 damage over 30 seconds. That's just one DOT. While the priest's lone DOT can do similar (but lesser) damage over 24 seconds with talents at its highest rank, it's the only DOT they can use unless you're undead and get the devouring plague spell (which has a really long cooldown, so realistically it's just not going to be used much). So just that one DOT is > the priest's DOT, and the damage on it is very, very significant. Then there are immolate and corruption and you know how those work. When you put all those together, the warlock is doing around 2x damage/tick from the priest. That's fair because the priest really doesn't have much in the way of offensive spells (though what they have is strong) and it's such a mana hog for them, while for warlocks we're expected to stack DOTs.
Then when you get to group roles, especially at higher levels... well, a warlock's main stat to focus on is stamina, so they get tons of HP. They also get virtually no downtime because of their DOTs and utility stuff like life tap, drain life, dark pact (talent spell but I really liked it in closed beta) that makes their HP not just more damage to take before dying, but a bigger extension of their mana bar in a sense. In addition to this, they can self heal with healthstones (which are seriously good - not as strong as a health potion of the same level, but very close, and you can essentially make an unlimited supply of them) and drain life (which is also so fast it's a very good way to contribute damage to the group). With all of this, you add in the above about CC and DPS and you may see what I mean about the class bringing a lot to the table. At higher levels, there are also parts of instances where AOE spells are important (these are areas where you fight a mix of elites and a swarm of non-elites, or even just all non-elites). While a warlock's basic AOE spell isn't as good as a mage's (and mages get 3 to the warlock's 2), the warlock can fill this role very easily. Plus there's the hellfire AOE, which you may have tried out - yeah, it damages you the same as you damage the enemies, but you can block the damage on it through a few different ways: voidwalker sacrifice, spellstones, and getting a lot of fire resistance equipment. It also makes group pvp interesting since you can be like a kamikaze attack of sorts (just saying that probably caused a bunch of people to go up in arms about being PC on the blizzard forums). There are also some higher level spells (some of which I never really got to test out, like soul fire) that are just crazily fun and strong.
Basically, warlocks have so much utility, which I like (and is valuable in WOW), and it makes playing them solo or grouped a load of fun because you work with managing aggro, CCing, etc. and still bring as much DPS to a group as the more straightforward DPS classes. I like it because you can do stuff that I haven't really been able to do with any other class, like chain pull guys with the voidwalker and DOT them up, fear juggle, or take on a group of mobs using the CC abilities to isolate and pick them off one by one. It just feels more tactical to me than playing a hunter or mage (I can't comment on rogue, only played it up to level 4) if only because of all the DOT management, aggro management, CC and so on.
I'm not saying they don't need improvements to the felhunter pet or curse of weakness and curse of recklessness, but those are things I can really trust them to get right. Enslave demon bothers me, too, but if what I see in the patch notes about curse of shadows is what I think they're doing (which is making it have a more tangible effect on enslave demon) then I'll reserve judgment there.
and honestly, I can't think of many classes I've played in WOW that doesn't have some abilities that just aren't all they're cut out to be - shamans with lightning bolt and some of the stat buffing totems, warriors with skills like shield bash and thunderclap (although maybe they improved that), priests with mind control (although this spell is SO fun in pvp), druids with their cat form DOT finisher... and the warlock spells we both agree suck. I guess I just don't mind that not every warlock ability is spectacular, because what seems like the core of the class (CC and DOTs) works really fantastically, and the other things they can bring like healthstones and soulstones (when that bug you've mentioned is fixed, at least) make the class highly valuable to groups, very strong soloing, and just diverse all around.
I don't know how else to put it, and half of this probably sounds like fanboy drivel, but I just find the class a lot of fun to play. It's like being a pet class in other MMOs, but faster paced and with these other utility abilities that make the class go beyond the typical. Not all of the game plays to the warlock's strengths, but I seriously find them a lot of fun. I solo, though, so that's probably part of it. Although, honestly, if you were duoing with a shadow priest instead of a paladin, you'd probably like the class more (that combo is near perfect for each class).
I don't think the class itself needs a lot of buffing, but I do think that curse of weakness and curse of recklessness need work, along with enslave demon. It's just that those things don't break the class for me since nobody, including warlocks, really debuffs all that well. I mean you can give warlocks the gold medal for debuffing in WOW and you and I both know how much they suck at it... but for me, I just like messing around with demon pets, doing tons of sustained damage (which allows me to focus on more than just if I have the mana to keep spamming a spell) and having the CC to make encounters a lot easier. Debuffing, at least with curse of weakness, is something they'll HAVE to fix simply because it's pointless at high levels (along with the shaman's stoneskin totem, which applies a buff that essentially gives the same effect) but even without it, I just use curse of agony on melees and curse of tongues on casters. Probably the best thing to compare this class to now is the Controller archetype in COH (based upon what I heard about that class at least) although warlocks aren't a mass CCer. It's just that they do the best sustained damage in the game, have the strongest AOE (although it also damages yourself and is point blank) and can CC the majority of stuff in pve, along with healthstones. I'm patient enough to know that Felhunter buffing will come, curse of recklessness will be retooled into something useful, and curse of weakness will be changed/buffed so that it's a viable debuff. But that's because I lived through the class at its worst when their DPS was horrible and compared to nobody, all their pets sucked (and they didn't even HAVE the felhunter) and there was literally no use for them except soulstones (which were an item handed out so they'd just be sold to other players).