Everything you need to know about Guild Wars 2 (for facts and questions about how the game works and such) can be found on their website and wiki:
https://www.guildwars2.com/en/
As for my experiences: Due to the timeframes I play, I tend to play this game purely as a soloist. You can actually level just fine without having to participate in any group content (at least up to the 30s; I haven't gotten higher than that), however, you start to miss out on a lot of fun. The group events are random (though some low level ones occur a little *too* frequently) and are a lot of fun and provide a lot of xp as well as a chance at some really nice loot.
As a side note to this: Do not be afraid of other players in GW2! Unlike WoW they cannot steal your kills or your nodes as everything is shared. That tree you see to go cut down can also be seen by everyone else so you might see 5 people cutting down the same tree and they'll all get their fair share of wood. So even if you are questing alone you're never truly alone if you see other players out and about because they can help you or you can help them and all without needing to be grouped up unless you so choose.
On a personal note, I find GW2's NPC world to be a little more engaging than WoW, but that's just me. I feel that the NPCs around you actually feel like they're living and breathing, but maybe that's only because a large portion of them are voiced and not just text based. I dunno, I just feel like the whole world is more alive.
If you're looking for a hunter clone, GW2 is similar, but, imo, better in a few ways. Sure there's no shiny, totally unique pets but meh, I like not pulling my hair out to tame that uber special rare that everyone else wants to have. You walk up to any pet that is green (friendly) and says Juvenile _____ (_____ being the type of creature) and hit the F key, and voila! you've made a new friend. You can also name your pets, though they'll lose their name after being store in their "stable." All of your pets are available on the fly via your K button and you can swap any 2 to be active. One of my favorite aspects of GW2 rangers is being able to play as a melee hunter. It's totally freaking badass and does a truckton of damage if you choose greatsword. Or use a 1H sword in your mainhand for a more defensive playstyle. Or play classic ranger with either the longbow or shortbow. Most ranger playstyles are fairly fun, I have found.
That's one of the other things I enjoy about GW2: Your action bars are not cluttered with abilities. You have 1-0 with the ability to swap between weapon sets (unless you're an elementalist and I think engineer) along with your F1-F4 keys (depending on class). You spend a lot of time auto-attacking, but it doesn't feel boring because GW2 requires you to actively react to combat. If you're the kind of person who likes WoW's point and shoot playstyle, GW2 may not be for you. The combat has a little bit of a learning curve, but it's not nearly as bad as TERA. It's a good in-between of WoW and TERA. You do want to try to be moving and dodging things when/if you can.
There are achievements, they're similar to WoW but not exactly the same. You can earn a lot of little small achievements that reset on a daily/monthly basis that you get credits for as well as regular type achs but I don't think they really do anything (or I have yet to get any that are important). There are titles in game but I dunno how to get them as of yet. There are no mounts in this game. If you are a die hard fan of mounts, this is not the game for you. You do a lot of running, but it doesn't feel like a lot because of your speed, speed boosts, plus the Waypoint system (which you can read about on their website/wiki).
I can't answer about end game as I do not yet have a level 80 character. To my knowledge there's dungeons, jumping puzzles, working on legendaries, and I'm not too sure about anything else or specifics.
There are no factions in GW2.
This game is really quite different from WoW, imo, but just similar enough that it doesn't feel too alien. However, I do feel that this game holds a bit of a learning curve that some will struggle with at first. I would not recommend rolling a thief, mesmer, or elementalist right off the bat as they have some of the higher skill caps (imo) as well as struggle with a lot of squishyness while leveling if you don't know what you're doing. My favorite thing about this game is that I do not need to pay a monthly fee to get quality content, and it is not pay to win (not at all). Yes there's still some bugs in the game but I feel like Anet is doing a pretty top-notch job of taking care of the game and providing quality content. Just these past few months have been filled with what they call living story events where they work on advancing different little arcs of the lore's story and you can choose whether or not to participate. You don't need to be level 80 for most of them and you get a chance at a lot of nifty looking stuff.
For me the graphics have stuck a perfect balance between realistic and toony and the game is absolutely stunning whether you play on the lowest or highest settings. I find the lore and races engaging and I'm always wanting to learn more. I love love love love love that questing is not the way it is in WoW. I have grown so burned out with that style of questing and it is so nice to just find a heart within my level range, complete its requirements, and then be on my way without so much as having to talk to the "questgiver" if I don't wish to. I personally like most of the personal stories, though you have the option of skipping them should you so choose to just wander about to different zones and only do events/heart quests. Though around level 25-30ish the personal stories start getting a little too samey, but it hasn't bothered me too much yet.
Leveling is a wonderful experience in this game; I find it can be rather easy to sit down on one character and crank out 5 levels or so just from how engaged I can get doing the content.
Professions in this game take a minute to figure out (or do a quick google/wiki search), but I like them a little better than WoW's professions. The discovery pane can become insanely addicting, especially for cooking (though I highly recommend any new players avoid cooking; it takes a lot of money/karma). I like that professions actually contribute to your xp as you craft.
This game has its ups and downs like any MMO, but I think it's worth a try for people willing to cough up the $60 (my biggest gripe is that they haven't added a free trial type system). It has a lot of very neat qualities, and despite its problems I feel like it's totally worth my time investment and have shone them my support by buying some things from the in game store with real money (you can also use in game currency to buy from the gem store). I've found that I don't miss WoW as much as I thought I was going to.