At lower levels I managed to do reasonably well, despite having a CHA that was 2 points lower than INT, by using things like Eyebite and Witchfire. So what, if I wasn't doing big damage to the BBEG. I was knocking his attacks down by 6 at level 1, so that everyone else could concentrate fire on him for a turn. Once a day I was doing reasonable damage to said BBEG, slamming him into the Defender and melee Striker, then slipping him back in their direction until he made a save.
Yes. One attack per encounter you had a seriously nice encounter power that hoses solos. And once per day you got ... a daily. The other 75% of the time you sucked.
My biggest success, in playing a Deceptive Warlock, was in deceiving the DM into thinking that I wasn't contributing much to the combat. There were quite a few combats in which I took not a single point of damage, either by dint of being ignored or teleporting away from (blinded) danger.
So. You managed to not only convince the DM that you weren't contributing much, but to actively make the defender and leader's job harder by encouraging the DM to focus fire. W00T! If you aren't taking damage then you aren't doing your part of the job; unless you are doing something pretty spectacular to make up for it (see: well played wizards), you are dragging the party down. Taking no damage is nothing to boast about - if anything the reverse. And deceiving the DM into playing the monsters more dangerously (as you did) is not something to be proud of either.
So to sum up you convinced the DM you weren't contributing much to the combat and this made you contribute even less.
And for at-will suckage, I think that the Bard's Staggering Note is right up there. No damage *roll* so no bonuses, and low fixed damage. Yes, it's a leader, but it's still painful.
There are two seldom useful types of At Will - the weak and the (normally even weaker) situational.
A weak at will is normally useful - but not very. As a general rule they are either about equal in power to a normal basic attack based on their stat or lose a little and gain a little. Good examples of weak at wills are Careful Attack (or whatever the Ranger +2 to hit one is that needs to compete with Twin Strike) and ... Eldritch Blast (most of the RBA powers except Magic Missile fit this - but EB is one of the weakest of these). The thing about weak at wills is that almost no one takes them because they are weak. Except the Warlock who has to take Eldritch Blast...
A situational at will is normally weaker than a weak at will - but when it is good it is very good. Staggering Note, Magic Missile, and Brash Assault come to mind. Magic Missile does what it does (autohit). Brash Assault is only any good if you have nearby allies with basic attacks; otherwise it is
terrible. Staggering note likewise. But with those two powers when they work they are superb - the trick with those is to set them up before you use them, and difficulty varies. Eyebite is another such power - granting a useful situational boost if you have something to set up. However balancing this is that it's weak the rest of the time. And because of the nature of it empowering your
other powers, it's not a good power if you're not planning on an Encounter or Daily. Give the Warlock more encounter powers and even if the opportunities to get high mileage out of it become no more frequent, you can pick them without having to fall back on Eyebite.
This means that (without Hexblade Weapons) the Warlock doesn't have a single workhorse power that does a decent job for routine use at low level. One at will is weak, the other's situational and therefore even weaker when the right situation can't be set up (which is a lot of the time).