Why to play Vanilla again? There is so many reasons...
Agree with all reasons mentioned by OP and will add few my ones
- Strictly rules
Rules which will not be changed at any cost, without any concession to those who want easy mode.
Today WoW lost his essence because they broke all its rules in trying to attract the large and a bit dumb population at the price to lower quality and become trivial
- Much more RP elements in Vanilla
WoW in the beginning was nice role playing fantasy game with the greatest focus on slow and precise construction of main character with a great micro management.
Today most RP elements gone or they are marginalized and the main focus is on competition of damage meters in PVE and arena's ratting in PvP.
Interaction with zones is completely marginalized by flying everywhere
Leveling and and getting to know our character/alts is marginalized too cause tons of heirloom even mount on lvl1, leveling become turbo rapid and with bonus exp players leave zone after completed just 20% of quests.
Nice RP fantasy game has completely turned into a shooter from a 3rd person perspective.
- Inbalanced releasing of new contents
Since middle of WotLK, after new content is out there are vendors with "free old content gear" or similar so most of players dont have reason to play ever again previous content except for nostalgia, transmog or achievements.
- Inbalanced duration of expansion's tiers
Usually last tier got duration like all previous two or even three tiers together (ICC, DS, SOO).
- Social aspect of game during Vanilla and BC
With new middle WotLK features like LFG, players slowly started to lose connections between them and community took "easy way" to fill their groups with totaly strangers.
In LFD players can meet other players probably once in life and they started to lose respect each for other, many ninja cases emerged and sometimes unnecessary argues and rude words.
- Distribution of work by classes
In Vanilla was no class who is capable to do all or most job, was very strictly distribution of duties.
Today for example nearly every class can heal more or less, 5 classes can tank and 3 can combat ress, nearly anyone can do CC, CC which is dumbed to the ground where mobs from same grp we can sheep 1st, sap 2nd, trap 3rd, paralise 4th and there is no possibllity that 5th un CCed mob can notice nothing strange around himself... world of dumb and dumbiest...
These are just few examples but there is many other things...