If I may offer a hopefully less biased opinion, speaking as one who loves both: Give retail a go, but don't expect the same experience. Both are very fun, but in different ways.
I've been playing retail WoW since Cataclysm and have been obsessed ever since. Given I was a late comer, I regretted missing out on all the glory days I had heard about from my fellow raiders, hence why I'm here (and loving it).
Here's the breakdown as I see it: Vanilla's strengths are a large, very friendly and helpful community and an incredibly immersive leveling experience. Vanilla is also much harder, which you may find a strength or weakness depending on your personal mileage. My main draw to Vanilla was lore (I'm a big Warcraft lore nerd, reading all the books as I continue playing), and it has delivered.
Retail's strengths are more versatility and diversity in experience at the cost of a solid leveling experience. The old world goes by in a flash due to how fast leveling is (and has to be now that the cap is 100). You will outlevel old zones looong before completing the stories, and often you'll only touch them before moving on to the next. That said, if you're willing to slow down and stick around in a zone after outlevelling it, post-cata most zones have new stories, and I found a lot of them enjoyable, which makes it a shame they blow by so fast.
Retail offers casual content in spades: Pokemon-like pet battles first and foremost. Pet battles are 100% optional and have no impact on the main game. I enjoy it as a side distraction, but if you hate the idea, it's easily ignored. Questing is extremely casual -- You will not die levelling, period, which sadly leaves many unprepared for their roles end game. Levelling has been revamped to sadly be a single-player experience all the way to level cap.
Levelling new content is a different story -- It's still easy, but the stories are fantastic. I loved Mists of Pandaria and the Warlords levelling 90-100 experience was very theatrical in its story telling and nods to old lore.
Retail is much less grindy than vanilla -- another strength or weakness depending on your preferences. As a married adult with a career, I appreciate the lack of grinding. I want a challenge, but not busy work.

On the downside of this, professions are, presently, all but worthless and entirely possible to ignore.
The worst thing about retail, IMHO, is the community is gone. Seeing how vanilla plays, I suspect it's largely due to the newer dungeon finder and raid finder functionality which pairs you with random people within minutes for any instance, then teleports you in from wherever you are. There was something to be said about having to travel to the entrance to a dungeon/raid and meet up with your folks there -- it added to the immersion. The randomness of the finders and the massive drop in dungeon difficulty means lots of rude kids cussing you out all along the way, or complete silence as you blow through. The sense of community is dead.
With that understood, *if* you have a group of friends to play with, this saves the experience. I started a raiding guild with my own friends and we are our own active and fun community. If you can find a good guild, that's where the saving grace is.
Lastly, there's raiding -- raid finder versions of raids are very much "easy mode", but if you have a large enough guild to raid normals, heroics, or mythics, those are a blast and quite challenging (either that or I'm a horrible raider who's easily impressed

). The rewards are not as special as they used to be though -- Your hard-earned purple gear from you guild raid is nothing but a color-swap of the exact same gear from someone else's raid finder run. Even worse, raid finder more or less has to be used repeatedly in order to gear up for normal mode raids.
So that's the gist of it as I see it. I love both and play both. I enjoy strengthens each has that the other doesn't, and I think that will keep me playing both for quite some time. That said, I 100% understand why a lot of vanilla veterans hate modern WoW. They have valid complaints.