Soul shard mechanics were messed up since they cam up with the idea of making haunt cost a shard. I don't remember if it was in Cata or MoP. In MoP it was already messed up. The Soul Swap mechanics were hilarious though.
I didnt play in MoP. Was it better in that expansion? (and im not asking that was it better without the soul swap 1 soul shard cost, cuz it was obviously better/op)
Overall, I think Warlocks became worse and worse from a "fun" point of view with every expansion since WotLK. Just some things that added depth to the class without neccessarily being OP.
- mana drain, I sooo much want that back
- friendly dispel on felhunter
- shadowflame glyph
- Fel Flame (was it the name? Instant cast that refreshed UA)
- baseline HoT and Death Coil
I would give up all those shitty new talents (SB:Haunt, Blood Fear, Unbound Will, ... whatever) to just get those things back.