Lemme take my own stab at it:
How does PvP work on L2?
You can attack anyone at anytime outside of towns, and they can attack you. Whenever you attack someone who is not in a war with you, you flag (your name turns purple -- note you need to hold ctrl to do this). If you kill a purple, there is no penalty at all. If you kill a white (someone who didn't attack back) your name becomes red.
When red, guards will attack you, and players can attack you without themselves flagging (meaning killing in self defense gives you more PK score). Once your PK count is > 5, you can drop items on death. All death outside arenas gives the same XP loss (except sieges and clan wars which are 1/4 of a normal death).
I hear the PvP is hardcore, and I'm curious how hard it is to level later on. If someone can simply camp you all day effectively reducing your exp continuously, how do you manage to level?
The only reason that that would really happen would be if you're in a clan war, or the person had some grudge against you. Camping corpses really isn't a big issue in L2 though, mostly they hit you and then vacate before your clan/friends counterattack them. If you're in a clan that has clan wars, your clan should also be backing you up on that.
Random PvP is rarer than WoW unless you're in the major alliances -- in WoW the game mechanics force people onto two sides, with nobody neutral. In L2 player politics create sides, and so you choose who you join, and you can be neutral or switch at will (assuming you can get into the clans). There's very little random attacking going on once you're past level 40 (virtually all of the random PKers stay and prey on the lower levels).
Leveling past 20 in WoW was almost impossible unless you grinded mobs in a secluded area or hid in an instance.
It's not usually a huge issue, again until you join a major side with a lot of enemies, but you usually wait until later in the game to do that.
How do you get a decent guild?
In WoW I was able to get a guild in 1 minute, and usually it was a bad guild.
You can try looking on the boards here or shouting in towns.
How welcomed are newbies?
Very welcome, actually, since academies were introduced. Only players below 40 can be in a clan's academy, and when they hit 40 and complete their second class change, the clan gets points they can spend to increase their level or provide benefits (like more defense) to their clan members.
Are you constantly bashed by being ignorant of quests, classes, or what not?
90% of the players are ignorant of quests. Although there are some gems and I've had good experiences with a number of quests, there are a lot of others that just plain suck. Many people only do the important quests (class changes, subclass quests, the quests to enter the boss areas) but you can always look on a site like
http://lineage2.stratics.com/ and check out their quest guides for things at your level -- there are often quests you could be doing. I don't usually hunt specifically for quests but I take the quests for the area I want to hunt anyway.
The chat system seems to be very uncontrolled, what is the general behavior of the players?
Like elsewhere on the internet -- good, bad, and everywhere in between.
How do you get groups?
A clan helps. Also, going to group areas (e.g. dungeons) and shouting by typing something like !Level 20 wizard LFG may work.
Since everything seems to be time consuming in this game (1 hour boat ride, 15-20 walks) is getting a group even possible later on?
Eventually you'll be using the teleportation system a lot more, when you're not so terribly poor like you are at the beginning. Typically you don't need to walk much (there are exceptions, like climbing 10 floors of tower of insolence...).
Also I was wondering if people invite you to a group a lot just to set you up and kill you while fighting mobs?
Maybe, but they don't gain by doing so. Used to be if you died to a monster you had a chance to drop items, so people might try to get you killed for gear, but that's gone.
How is the economy?
Something that you absolutely need to learn to deal with. Money is something you will never have enough of, and you need to work on a plan to get the gear you want. Unlike WoW your money and item drops won't nearly cover your expenses. Quests, manor system, making a dwarf alt, playing the market, there are many ways to earn cash and you better find one that works for you. BTW remember this -- Giran price = actual price. Newbie towns are way, way off of what items are actually worth.
Do not EVER sell a base material (iron, stem, varnish, etc) to the NPC shops. Most of them are worth triple at the very minimum. Until you get to giran, save them. For recipes and key items (e.g. "Sword of Revolution Blade" is the key item for a SoR), a good rule of thumb is "if the NPCs don't sell the full weapon/armor the key item is for, keep it, else sell the key item to the NPC".
The economy is player-driven since, after C grade gear, nothing is obtainable except as very rare full drops or being crafted by players. The "economic grind" is as important as the XP grind, so prepare for it and make a plan.
What are the desired classes?
-Nukers and archers are really good at PvP.
-Dwarves are good at making money.
-Buffers like warcryers, shillien elders, and prophets are very needed but often dualboxed as the play style can be boring for most, plus the dualboxers usually keep the buffer out of party for better XP.
-Bladedancers (and to a lesser extent swordsingers) are very much needed
-Melee damage dealing classes can often do well and make up the bulk of groups, but they're not as strong as the ranged classes in PvP, especially in small group PvP. Some of them like Gladiators can be pretty good anti-archer.
-Tanks and healers are strongly needed in certain areas but in other areas they're unnecessary; they're mostly best in large parties not small.
I don't want to spent time leveling up a class that is useless or undesirable or too common (rogue). I don't like being a "dime a dozen" class but I want to be able to solo (ex. not possible for a "holy priest" to farm)
Few classes can solo all the way up. Summoner classes can, and archers/nukers to a degree (though nukers really would want a shillien elder to buff and recharge them, so most dualbox one -- sorcerer and spellsinger are particually bad at keeping their MP up.) Many classes can solo with a source of buffs, but few can do it on their self buffs, at least not all the way.
In contrast, duos and trios can be very good XPing.