I think part of the reason it feels off today compared to then is the Dark Archangel vs Voidform. Voidform doesn't feel like a proper cooldown; it's so core to using it in the rotation these days that it's not exciting when you get the buildup into it vs Dark Archangel which felt like a proper cooldown back then. Losing SW: Death as a baseline ability that shadow has had since I believe BC is also very awkward; taking something that has been in the shadow priest kit as a natural ability for close to 12 years and making it a talent that isn't even a strong one just feels wrong. Cataclysm also allowed you to play like a proper spellcaster if you wanted to with Mind Spike/Mind Blast instead of being DoT focused, and they had fantastic set bonuses that either changed the way you played in between patches or encouraged proper play (such as reduced CD's on Mindbender/Shadowfiend and buffing Mind Blast damage if both DoT's were up on Firelands tier) or the Dragon Soul set putting a lot of emphasis on SW
eath.
There was a real feeling of cooldown usage back then and having set bonuses made you feel much more powerful once you completed them, in addition to Mind Blast hitting like a truck back then to make up for the lack of burst damage priests had overall. In addition, Vampiric Embrace felt like a legitimate raid cooldown back then compared to something that is basically overlooked these days. In addition, orbs were a thing back then but gave a significant buff to Mind Spike or Mind Blast damage back then; stacking up orbs and lining up your burst with them was a skill-based thing to do if you could line it up with trinket procs, Dark Archangel, or various other cooldowns available to the raid. Mind Blast went from a filler spell that barely did more damage than a Mind Flay fully channeled to a significant part of the rotation, and at the time in Cataclysm also had a Mortal Strike-esque healing debuff built into it, making them a much more viable class to pick into a PvP match as well. Strong damage over long periods, slowing on Mind Flay, healing reduction and high burst on Mind Blast with Dark Archangel, and the durability added by using Inner Fire for armor and spell damage and swapping when necessary to Inner Will for the movement speed increase was a nice bit of flavor which I used in a macro to alternate between the two since only one could be active at a time. Inner Will swap while moving and back to Fire when stopping was another way to show skill at the class but most of the things you can do to show individual skill has been removed other than not letting DoTs drop off.
Cataclysm was the peak of shadow priest for myself for sure.