Honestly, I don't think DOTs could be buffed on the class at high levels or it would actually make us even less useful in groups, because we would constantly steal aggro from the tank and that just makes us a liability vs a mage.
It's why I really think an approach to make us more like a cross between the hunter and the mage (pure nuking for offense, pet to tank or DPS depending upon the situation) would balance the class even if it would water it down into solo pet and group pet (though blizzard WANTS THIS TO HAPPEN ANYWAY, according to all they've said throughout beta).
I can understand what you're saying, though, cause it's the most obvious way to buff warlock DPS (which doesn't really need buffing as much as the other problems) and the debuffer role. But I know enough of how hate/threat works in this game that if we were able to stack DOTs with impunity, the class would devolve into either a snoozefest of DOTing and sitting back or be a gigantic aggro stealer. And at high levels, only raid mobs can actually be alive long enough for most DOTs to go full duration. The sad thing is, I think even if you ignore the debuff limit, the best way to DPS with the class is to simply curse of shadows/curse of elements and chain cast shadow bolt/searing pain (first setup is for group, second is for solo). Which is so boring... and so not the way the class is supposed to work. If we just had other reliable fire/shadow nukes beyond shadow bolt and searing pain (which we can't use in groups at all) we'd go a long way towards having more variety and, IMO, better playability. I don't think it would unnecessarily dumb the class down either, because it's pet management that's the hard part, the DOTs just require you to have their durations memorized (while soloing) or to count ticks of them.
Soul shards, I definitely think need to be removed entirely. Every spell that uses them is balanced in other ways without them, so it's just a tedious addition. Especially at level 60, where if I need to get shards to pvp, I need to go to a place like western plaguelands. Blizzard's proposal of a shard bag does nothing to address this, and the drain soul in pvp thing helps a little, but it's like putting a bandaid on something that should be amputated.
I definitely think that the endgame pets should actually be permanently controllable, just without any special abilities, and those abilities would be added in as the rare drop grimoires instead of the ones that teach you the summoning spell. The Infernal is literally useless because there is no situation at all where he's worth the risk over any other pet. The doomguard is very very useful but too risky (ironically enough he comes with dispel magic, so we get to counter our own class with our ultimate pet). His debuff spell is so strong that it almost makes me angry that our own class doesn't get it, when the "best" pet does.
I also feel that the dispel spells should be nerfed. I think the suggestion you had before about making them scale mana costs with levels, and adding in a small (like 4 seconds) cooldown would be perfect. Fear's new mana cost isn't a big deal (it's 205 at level 60), it's just like insult to injury. And personally, Cleanse should be nerfed into oblivion and made into like 3 separate spells, because it's the idiot button for dispelling debuffs. Either that or other classes that can dispel multiple types of debuffs (the other 3 healers) should get a similar all-in-one ability so they have less hotkey space taken up.
As far as curses go... yes, they really are pretty weak at 60. Agony is one I only use if someone is offtanking a mob since it can soften them up some while the main mob is being destroyed. The resistance curses are great, recklessness doesn't have any real strong debuff effect, but the no-fleeing effect is good. Weakness got nerfed for no reason, which then led to shamans' stoneskin totem getting nerfed (when both already scaled terribly at high levels, and now scale terribly past about level 12). Sadly, I've found that curse of tongues can be a liability with some high level caster mobs, and it doesn't even seem to slow the casting of some spells (most notably the summoning spells scarshield warlocks and blackhand summoners use in BRS). If this were replaced with a spell interrupt it would make the class a lot more versatile and wouldn't overpower it really (because ANY class can solo caster elites that are outdoors, but in instances this is just yet another weakness of the class).
Ultimately, I think we need soloing nerfs and grouping buffs if that's what it takes to get this class fixed. No class is that hard to solo in this game (save maybe a holy spec priest) and most players know that having a worse solo game doesn't mean much when you contribute a lot to groups at level 60, whether it's pve or pvp.