I'm going to speak for Shadow, from Cata onwards to WoD - these are all off the top of my head, if I forgot something, don't sue me:
- lost Renew ( insanely useful button to press )
- lost Heal ( whatever, no loss here )
- lost Binding Heal ( great loss considering this was awesome versus affli lock and rot teams in general, to help out your healer while you were also low-ish )
- lost defensive dispel ( RIP wotlk, hello cata+ idiocy where shams and druids can dispel. I'd be okay if we could just dispel ourselves, like we still could in Cata post-dispel removal - it's super retarded to suddenly lose the ability to quickly dispel shit like frostbolt slow off yourself, just fuck off )
- lost Mind Soothe ( RIP, best grounding/reflect eat spell EVER )
- lost Mind Control ( coz same tier as Psychic Scream )
- lost Inner Fire/Will ( I actually liked swapping between these two )
- lost Cure&Abolish Disease ( a button I really miss sometimes )
- lost Holy Nova ( can't aoe idiots out of stealth anymore, coz mind sear requires fiend/partner near you )
- lost mana burn ( can't ever get over this tbh, as this was THE button to sometimes press when games were going nowhere and it was a complete stalemate when it came to strength of teams - this caused SIGNIFICANT pressure on enemy healers )
- lost prayer of healing ( whatever, nobody ever used it )
- lost Divine Hymn ( a great offheal tool )
That's 14 buttons lost, 12 of which added a great amount of depth ( IN MY OPINION ). I've used most of them, OFTENLY.
How do you think I feel when my globals get significantly limited? So instead of having to choose between 5 different spells for that global, I can only really use one ( if that ).
That's why ( USUALLY ) more buttons == more skill. Like, Prayer of Healing and Heal added no depth whatsoever imo, but all of those other buttons added depth. Were they spells I can live without? Obviously. Did they add depth and actually made the game more complex, and allowed me to distinguish myself from other players of my class? YES.