Destiny for MMO and Halo Lovers
Classes. Class means everything in almost every MMO you'll play, but really, I don't think you need to worry about what class you play in Destiny. I don't see much in the way of any class being super over powered and since there is no Tank/Heals/DPS paradigm to adhere to, you can be who you want. The easiest way to look at it is where you like to fight from. Titans fight up close and personal. Warlocks are a little squishy but can do damage from afar and offer buffs to their teammates. Hunters are a middle ground and are focused on precision shots over shotgun blasts. It's all about how you like to play.
Patrol. Every planet has a Patrol mission. These are like the open world of your favorite MMO. You roam around and kill baddies while picking up small quests that ask you to kill certain things or to run to a specific place for something.
Tower. Orgrimmar if you're Horde or Stormwind if you're Alliance. It's the capital city. You'll head here after a couple of missions to turn in bounties and decrypt gear.
Cryptarch/Engrams. I can't think of an analogue to World of Warcraft for this one. Engrams are encrypted piece of gear or weapons that drop after killing enemies or handed out as random rewards at the end of a Crucible/Vanguard game. You will get these often. You turn them in to the Cryptarch at the Tower to find out what piece of usable gear it contains.
Vanguard. PvE. Player vs. Environment. Your traditional baddie killing mode. Co-op or single player.
Crucible. PvP. Player vs. Player. It has it's similarities to traditional Halo multiplayer. All of your attack and defense stats are normalized in this mode, but your upgrades (that don't deal with increased attack or defense) still matter.
Iron Banner. Similar to the Crucible, but without evening everyone's gear stats. If you go in with a level 2 auto-rifle, you're probably going to have a bad time.
Bounties. I'd relate these closest to dailies in World of Warcraft. Both Crucible and Vanguard variants exist. They are handed out by a robot at the tower or you will sometimes get a message from the Postmaster (but you still need to get the bounty from the robot). There is a countdown timer to when they reset (which currently appears to be at 4AM CST). They give you a pretty large XP boost so I recommend you always have some on hand. You can only carry 5 at a time, so managing which ones you have on you at one time sometimes becomes challenging.
Light. Destiny has a soft level cap of 20 at the moment. After you reach level 20, you are able to nudge that level up by obtaining armor with the Light attribute. The more Light you have on your armor, the higher your level goes. I compare this to the World of Warcraft item level (ilvl). After you reach max level in WoW, the only way to really tell how powerful someone is at a glance is to look at their item level. This is just represented differently in an artificial boost to your actual level in Destiny. You will be required to boost your level in order to be effective against higher level enemies.
Crucible Marks. Honor or Conquest in World of Warcraft. This is the currency awarded for doing Crucible (PvP) things.
Vanguard Marks. Justice or Valor in World of Warcraft. This is the currency awarded for doing Vanguard (PvE) things.
Strange Coins. I'm not sure if there are other random currencies, but the Strange Coin is used to buy things from a vendor at the Tower who only appears on weekends. He's located on the left side of the tunnel that leads to your classes representative.
Control. It's closest relative is something like 3-Plots in Halo, but with a twist. Control still has you trying to maintain control of various points (3 of them) on the map, but it changes the way you score points. In Halo, points were scored over time by controlling the points. In Control, no points are awarded over time, but each spot you have control over grants you bonus points for each kill.
Clash. 6v6 Team Slayer in Halo. Straight up guardian killing.
Skirmish. 3v3 Team Slayer.
Rumble. Free for all, 6 player Slayer.
Strike. Dungeons from World of Warcraft or most other MMOs. These are limited to a 3 player fire team with matchmaking available to fill your team if you're short.
Raids. Although raids haven't been made available yet, plenty is known about them. A 6 player fire team is required. They are the end game content in Destiny and will require a lot of skill and coordination in order to complete. It has been stated that raids will not inform the players about where to go or what to do, but rather just let them figure it out for themselves. Because of the coordination required, matchmaking will not be available so you will need to have 5 other friends who are all around the correct level to participate. I'm unsure if you are capable of doing raids with a fire team that's smaller than 6 players.
Strike Playlist. After you unlock each of the different Crucible types, you'll unlock the Strike Playlist. If you play WoW, this is a pretty nice way to grind strikes (dungeons). You get thrown into a random strike with an increased difficulty and when you finish that, you queue up for another one.
You'll also want to understand how your weapon's stats correlate into damage done. The two important things to consider are your Attack vs. your Impact.
Attack. The attack value on your weapon is showcased in a large number at the top of your weapon information. This value is directly related to the defensive ability of the enemy you are shooting. This number is a lot less meaningful when shooting enemies who are lower levels than you.
Impact. The Impact bar in your weapon stats is the actual damage that a weapon will do on a per-bullet basis.
Attack + Impact. basically, if you're level 10 and you are attacking a level 12 enemy, your Attack power will come into play and help you do a little more damage to that enemy. But if you're attacking a level 8 enemy, you might find that Impact is the stat that does the most damage to this enemy. Bottom line, you want a high value for both of these. This video does a pretty good job of describing the difference. Obviously, Rate of Fire, Magazine and Reload combine as well to compute your overall DPS.
A few last minute tips, as well.
Play with friends! Can't stress enough how much fun I've had playing this game with friends, and I'm normally one who really enjoys playing games alone. Both Crucible and Vanguard are a lot more fun when you have a full fire team of friends along for the ride.
Don't play the story like you would in Halo. Story mode in Halo was always something I turned the lights off and immersed myself in before I did anything else. Destiny's story isn't really made to be played like this. You're really meant to explore and patrol between missions and take your time. Playing with friends doesn't hurt the story either. Cutscenes are really short and you can't skip them. So if you're new to the story, you get to experience it. If you've already done that mission 10 times, they're short enough to not be annoying.
Explore and have fun! I've noticed that everyone is able to play the way they want to play and still have a blast. Hardcore PvP guys can level up and do everything in the Crucible, while people who'd prefer to explore the world and level up by shooting Vex in the stomach are able to get there that way. Destiny does a great job of letting you play however you'd like, so take advantage and find the perfect way to play for yourself.