With a standard subscription, you pay a monthly sub and get to access all the content outside of full expansion packs. The only additional RMT is usually just cosmetics or stuff like server xfers, level boosts, race changes, etc.
With F2P or Buy-to-Play, you usually see a couple approaches outside xpacs: You either see the predatory "mobile game" model, or you see the Barbie Dress-Up model in terms of RMT.
I think people mostly have an issue with the mobile game model, because usually that means they are intentionally designing mechanics or features of the game to be as frustrating as possible to encourage people to pony up cash to get "convenience". Like a lot of Korean MMOs - they make it so you need good gear to PVP or PVE, but then design bullshit crafting mechanics that have a chance to blow up your gear unless you buy some resource off the cash shop (and even then it might still explode). Or they limit your ability to participate in content unless you buy something that lets you bypass a long timer.
Archeage was the fucking worst - not only did you have to pay a sub to get base features like home ownership or labor regeneration, but then you STILL had to buy labor pots to do anything in a realistic amount of time, or tax certs to keep your house, or crafting charms and scrolls from lootboxes to -maybe- prevent your gear from exploding when trying to upgrade it. Or they would nerf boats or vehicles in the game - then they release an unnerfed version in the cash shop you could buy for a ton of money.
In this, you have access to ALL content (the story is actually pretty fucking good for a foreign company, honestly. Translations are really well done). The "questing" systems plays every bit as much like a classic CRPG with backstabbing, politics, "player choice", etc. Like I said, it's surprisingly good. Sure, it's no Baulder's Gate 2 or Planescape: Torment, but it's every bit as good as something like Divinity 1 and 2 - storywise.
Then of course throw in your standard MMO shit - there's a "task" system where you can get a repeatable quest to kill things, turn it in for a big chunk of exp/gold, and then you can't get another one for 15-30m depending. It's basically there to supplement your grind/questing. There aren't really traditional "hubs", per se. You basically have the "main quest", then as you level you'll unlock "side" quests which have their own storylines - some of which are silly, some suck, and some are actually really good.
The combat is mostly a gimmic, but it does have its charm. Missing 3x in a row on an 81% chance to hit sure is fucking fun though and is enough to make you rage when the mob kills you on the next round - very XComish.
The big issue right now is party play kind of sucks. You can join a group, turns are simultaneous and you have 10s to make your combat "choices" each round, but ONLY the party leader controls where the group moves, mobs it fights, etc. You also can't see any quest dialog of the party leader/do your own. Essentially, it's mostly just there for grinding and/or killing tough mobs. It's GREAT if you're playing with friends you trust, but if it's just randoms, they can be pretty potato.
But yeah, like
Hateyou
said, not paying for premium would be like playing EQ for "free" but you have an exp/plat penalty. You also apparently rest(most fights require you to rest 10-30sec after) faster with premium, but I haven't noticed much of a difference between premium and non, tbh.