This is not WoW, it is D&D, if you're not a team player you will fail. You will have a lack luster experience.
To be sure, there are a lot of MMO caveats, like the Auction house, chat channels (not much barren chat, but more now with f2p), Looking for group, looking for more, PvP, content alterations for rogues (in PnP rogues have versatile social, stealth, & recon roles which are all but lost in an MMO), real time combat, etc which essentially you would never find in a table top game. Because of this there are other changes too, level restricted items, no evil characters, changes in duration of spells, more mana points, less opportunity to rest, no Friendly fire, really high trap & lock DCs to keep rogues useful . . .
A word on PvP: Most MMOs take a lot of time to "balance" MMO classes, this is not the case in DDO because class balance is subtle. Mages (Sorcs/Wizards) can generally 1 or 2 hit anything in their level range, this isn't "Fair" and it's not supposed to be. Clerics in DDO aren't squishy, they are in fact one of the most powerful classes around - and they're supposed to be, to make people want to play them. Rogues are about 2 seconds from sucking, but they can disarm traps and open locks - making them painfully necessary. CC is totally Over-powered, like Hold person which not only drops someone out of the action for 15s but makes every subsequent attack on them in that time a critical hit, and why a "priest" will destroy a "Warrior" in PvP everytime.
So while the Classes aren't "fair", they do need each other. DDO has hostile collision, which means the "tanks" can block enemies keeping them from getting to the rest of the party, which is great since the Mages can generally be 4 hit by anything up to 4 levels below them and you'll be wanting them to 1 hit your mutual enemies fist. But you'll need clerics to heal you while your blocking, and a thief to open the treasure chest since there's no key. It's a team effort because D&D is a team game.
So you're kinda stuck with everyone (on hard/elite/epic), which is great if you're all team players and know your roles. And abysmal when some tool from WoW thinks he can tank without support, has a rogue DPS build ("he's an assassin!"), or decides he's going to go do his own thing. Elite tends to be a great test of this, a group 2 or more levels above the quest can easily be wiped if they lack teamwork, coordination, and a plan (Voice chat please!) while a good team (preferably who've worked together before) can often do elite instances above their level.
Also on the positive side everything is instanced. Not only do you not have to play with people who suck, you generally won't have to run to a new place for 20minutes. (Long transit times are the stupidest thing some sub-human piece of crotch rot ever put into a game, I'm talking to you Blizzard and the "MMO model". Buy a mount, how about making your game not pointlessly large?) Often different instances are < 15 seconds from each other, occasionally they're so close you can choose from several less than a second away, and less commonly they are a one or two minute run from each other. But I've yet to experience the dreaded OG to Thunder Bluff run, or even the ridiculous intra OG run (bank to black smith). OG feels large just to be pointlessly large. Really, my trainers have to be in the back? Why not right up front where we'll all use them? In Storm reach and DDO in general everything is relatively close, as it should be. Trainers, AH, and merchants, the "Houses" (like House K) deviate from this somewhat significantly (since you're accustomed to everything in easy reach) but it's still not OG. Logging out in the city should be way faster, though it's on par with other MMOs. I'm really not clear why game designers design so much pointless tedium into their games.
So those are most of the caveats. It's important to keep them in mind as most of the intimate feel of D&D is still there due to the nature of the instances. And keep in mind you'll run into a lot of douche-bags, and they'll ruin your parties, so don't do anything on elite with them or add them to your friend's list. In fact, I'd like a D-bag list to help remind you who not to group with (fortunately you can make comments on everyone in your friend's list to help with this).
You also control your level, so you can keep doing "low level content" with your friends and when they level up, just level up with them. You keep earning XP, you just don't have to level that character if you don't want to.
Generally no bag garbage (I have to take these 6 things with me to complete this quest? You've got to be kidding me. . . wait it's been 3 weeks, which 6 things did I need for this quest, and which 4 for this other one?.)
You also earn XP based on completing quests, not individual kills. So the grind is not about waiting for repops or running all over the map, generally. And since everything is instanced there's no kill stealing, in fact killing everything only gets you +15% XP.
Monsters generally won't just leave you alone if you run "too far" you're invading their home, so they'll follow you if they see you on patrol (to a point).
And the negative . . .
NPC view distances are shorter than yours, reducing the need for stealth. Not much in the way of dungeon alarms so generally the Big Bad will wait for you to find him in his lair after his minions are all dead (MMO status quo) rather than prepare a counter offensive (table top status quo). Which has caused some players to solo major instances via the "zerg" method. Unfortunately the designers implemented the "dungeon alert" system which causes a slow effect to curtail this otherwise legitimate play method. Dungeon alerts are terribly unfun and absurdly easy to trigger bespoiling many instances which have high spawn rates and imperiling entire parties due to a few simple missteps. This really destroys the D&D atmosphere which theoretically is the entire draw of the game.
If you must lone wolf it, you can buy a Mercenary, but generally even a crappy human player of the same class is better than any Merc. Humans are just better. And if you're a fighter or healing class you can get to level 10 easily enough without help and 20 with good Merc skills and lots of potions and scrolls. Mages and rogues can potentially have much harder times, but can make a go of it as well. It would be nice if you could hire two (or more) Mercs to combat this, but sadly you cannot (without a visit to the cash-shop).
Not all quests give rewards equal to their difficulty some, like proof is in the poison, are very difficult and offer very little reward.
The descriptors "Short" "Long" and "Very long" are not adequate. A "quick", "medium", "extremely long" and "Unbelievably long" (the Pit) descriptors would be a nice addition.
If you want to Role-play, you really can't. Much of the theater simply can't be recreated if you want a game to have mass appeal, so RP has been replaced with strategy which is also fun. I really hope Bathesda makes an Oblivion or Fallout with multiplayer & a persistent world (if they don't maybe I will). Role-playing in this game can be done with a dedicated group/guild, and I'd love to experience it but . . .
The guild system is crap, more guild tools are needed. Leader, Officer, and Member. Really? That's all we get?
It's Free to play... mostly. You'll really need to buy a module or two if you want to experience a fuller game (not to mention a joint bank) or progress past level 10. If you plan on playing for a long time, pay to play may well be the way to go. If you want to try it out for a few months casually, F2p it up. If you want to try it out for a month hardcore, VIP it for a month or two. Generally your characters will be waiting for you after your subscription lapses, so you can pick it up after your trip to Europe. Although after level 10 F2p & even some P2p groups become hard to find, since it all turns hard stop for F2p content after that.
It has many of the same economic problems as other MMOs, like gold and item inflation. But fortunately there aren't a lot of "Bind on pick up" items. So use an item for a while, then pass it to an alt or AH when you out grow it.
No modules, so no way to:
-gauge DPS
-other people's HP or magic points
-easily monitor AH prices
-alter the UI to suite you better
-automatically destroy collectibles you determine are useless.