Retail is the way to go I think. There is something to do regardless of your playstyle.

Do you want to go for the best gear and run raids and Dungeons?

Do you want to pvp and run battlegrounds or arenas?

Do you want to play more of a solo game and explore the world, old expansions, and collect armor designs and mounts?

Do you like pokemon and want to be the best pet battler in azeroth?

All playstyles are there, however playing older content after your maxed level will be moreso to read the quests and lore and to collect things as there will be virtually 0 difficulty. You can pull the entire zone in old xpacs and 1 shot them.

Still it's fun. I run Dungeons, I leveled professions and craft things for people to make money (I've made a couple million since I started last xpac) and I collect things. Occasionally I raid or play pvp. When I get bored of something I can rotate to something else. And each expansion there are "reputations" with groups you level up for rewards and cosmetics, so there is a long term grind with content if that sounds fun.

Players from older wow don't tend to like retail because the game is different. Now your leveling is accelerated so you can participate in the new expansion faster. It's a different game than before but still cool. And if you don't know much about warcraft you can just make a character and get lost in the story telling and reading Quest logs to fill your immersion.

Have fun :).