But probably not why you think. I actually think Retail is a bit of a train wreck when it comes to the actual game - I got to like level 15 in Mulgore and just stopped wanting to play because it was vomiting so much at me, so fast, and everything seems to be geared towards leveling your nth alt instead of a new character on a new server for the first time since Legion. With them trying to minimize the leveling process so much, I honestly wonder why Retail has it at all - why not just do a Guild Wars 2 kinda thing where character level is basically just a sort of achievement system and is largely irrelevant for "meaningful" content? I actually fired up GW2 for the first time in forever just to see if memory matched reality and... yeah, it's pretty neat how you can almost immediately access all of the "end game" content in GW2 (but *holy shit* they've become mega predatory with their freemium UPGRADE TO A FULL ACCOUNT bullshit in the time I've been away, I feel like I need a fucking shower after playing.)
But, no. What made me lose interest in Classic, after playing Retail, is how much *better* Retail looks and sounds. Animations are better, character textures are better and customization is better (I love all the little gewgaws tauren can add to their characters, the war paint or tattoos a lot of races get, etc), sound effects are better (Shield Slam sounds like actually slamming something not that weird DINK noise, and whatever the fuck Classic's awful Devastate noise is), etc. It really actually bothers me that those nice things aren't even an *option.* I don't think it's because of some adherence to NoChanges, I think it's just that it would take them some extra effort and time to make "modern sounds and visuals" an option for Classic, and Classic is operating on a shoestring budget.
Then, also, the automatic quest helpers, the better, new Dragonflight UI, on-screen proc notifications that don't require WeakAuras, etc. Like, I love the world and gameplay of Classic way more than Retail (although I think the new DF talent system is fucking awesome), to the point that I'm not really interested in playing Retail's disjointed, "shit piled on top of shit piled on top of other shit" world. But after even just an hour or two of playing Retail, I'm also *really* not wanting to go back to the ugly, archaic jank of Classic. It's weird, playing a little bit of Retail makes me not want to play Classic... but playing a few dozen hours of Classic makes me not want to play Retail.
Maybe I'm just wishing Classic had a larger budget?