The game is a one time purchase. No other monetary commitment is needed for the game's foreseeable life span. The design GW2 is fantastically interesting and is worth full retail price if one is concerned (as you should) with investigating game design across a variety of video game genres.
Population wise, GW2 is [anecdotal] the most lively game I have ever played sans World of Warcraft circa The Lich King. Factually, Guild Wars 2 is one of the best selling MMOs of this era. Concerns about population across servers is not relevant.
Gameplay wise, GW2 is an action game MMO. The learning curve is no greater or lesser than any post-World of Warcraft MMO. And about on par with other twitch & reaction based action games seen on the PC platform using the WASD control schema.
Class choice doesn't matter in my view. I find them all relatively the same. Maybe Engineer or Mesmer are poor choices to start off given they take a few levels to really get going by comparison to others. Though that is my own subjective view on the matter.
If you favor having a wide world to explore where content is always relevant to you because your level scales to the area, then definitely give GW 2 a try. There's plenty of events to participate in, the Living Story updates to check out, and numerous goals to pursue in terms of achievements and the like.
As for classes to start with, Warrior and Ranger are two of the better ones to jump out with. But don't be discouraged from trying them all out and pick what works best for you, or just play as many as you like.
For a one-time purchase you get a lot of things to play around with, as often or as seldom as you want. No sub fee means you're never tied to the game financially. If you want things from the Living Story content, you'll need to play while it's available to get it, but even those tend to be fairly easy to pick up.
I bought Guild Wars 2 and put it down after a few weeks. It's a graphically beautiful game with some decent features that sound great on paper, but seem to be lacking whilst actually playing.
For example, the movement is a bit clunky. Honestly, I feel liek no other MMO aside from WoW has been able to capture the same seamless movement and directional change of character as WoW has. Is it really that hard to have fluid movement?
The second thing is that there is no raiding. Sure, sounds good on paper, but once you hit the endgame you realize there is nothing different at 80 than there was at 20.
The world bosses are really cool looking and once again beautifully designed, but the encounters themselves are pretty faceroll and you get used to them fast.
The world v world is nice until you realize its one big aoe fest and you need a quad sli system to run it properly.
The questing system is another one of those things that sounds good until you realize once again it's just a massive grind with no real direction or story.
These are just my opinions on it. Others will disagree.
Caveat: These were my experiences a year ago. Maybe the game changed since then.
Edit: One of my biggest rules of thumb when examining a new game, especially an MMO, is to see whether or not it has turned into an e-sport. That typically indicates that there is a great deal of passion surrounding the game, so much that people are willing to pay to watch it and play to in money for it. Also, it usually means the game is generally entertaining enough to keep you behind the screen for hours wanting to improve. WoW, Dota 2, LoL, SC2, BF, CoD, etc. all have legitimate e-sports elements and are all solid titles. I don;t think GW2 has a competitive e-sports scene but I could be wrong.
Blizz just threw money at korea, but nobody really cares. It's also a poor test to weight a game by, because of the fact e-sports is mostly about the $$$ side. I wish players realized this. E-sports is a sham and bad for pvp players. It's for people who want to game for a living and are willing to play anything that makes them cash. It's not a judgement of the quality of the pvp. GW1 was before e-sports took off, but it's far more legit competitive game than any MMO. The whole e-sport thing is about self-promotion of nerd personas and corporate advertising. It's not about the game quality.
On the "casual" side...well, nearly everything lol. It's a very casual-friendly game in terms of not requiring you to be in a hardcore guild to get anything done, you can do a LOT just on your own or while playing with a friend or two.
On the "hardcore" side GW2 has a mega-gear-grind but only if you're min/max nutter looking to deck yourself with ascended/legendary shit. It's largely optional, you can run 99% of dungeons just fine with semi-decent gear as it boils down to skill, builds, ability usage, knowledge of fights/mechanics and just being a good player. There's no DPS meters and no inspect option, people will know whether you suck (or rock) just by how you play
The learning curve is...not so friendly. There's no tutorial to speak of, you're just thrown into the world with minimal guidance and are expected to find your way around, learn how the combat and dodging mechanic works. But it's quite easy to accidentally wander into high-level areas where the mobs will eat you, or find yourself in an awkward situation where you've finished all the hearts/events in the current zone, pushed through your story quests as far as you could, but still haven't leveled enough to progress to other zones. This is usually solved by going to another race's starting area and doing hearts/events there.
Speaking as someone who quit WoW and went over to GW2, the learning curve is very steep in the initial hours depending on the class you play. Just don't play an Elementalist if you don't enjoy dying a lot and learning things the hard way, that class is ridiculously complicated (for newcomers) with terrible survivability and little pay-off. It's NOT the equivalent of a Mage/Sorcerer in any other MMO in terms of nuking things from range and having lots of CC/roots. There are other classes in GW2 that do that far better, pick anything else.
Overall classes are quite flexible so just read their descriptions and go with whatever you like the sound of. GW2 is VERY diverse with builds/weapons available to classes, e.g. with Thief you can pick Shortbow / Pistols and become a pure range dps. With Ranger you can pick a Greatsword or Sword/Dagger and be a melee fighter. Warriors can do big damage with Longbow and Guardians can use Staff like a spellcaster. Confusing huh? You'll come around to love it
That is worth pointing out: if you're looking for some big endgame raiding content, GW 2 is NOT the game that has that. They do have dungeons and the fractals, but nothing that's like the raids WoW has. The trinity does not exist here. As others noted, it's personal responsibility and teamwork that will see you through above having super-awesome gear. What you do at cap is everything you did while leveling, just more of it is available.
GW2 is going hot and heavy. My server is not a top server and we constantly have new players coming in.
The learning curve is fairly easy if you have a brain. You can even use google to look things up, or go to the GW2 wiki! It is amazing, I have never seen anything like that for any other MMO EVER! Can you believe it? Web searching for things you don't know! It is a breakthrough I say!