Years ago, when I first played this MMO, I was turned off by the fact that you actually have to pay to play. This is considering that it is, as far as the multiplayer concept is concerned, exactly what Guild Wars have, only that Guild Wars is actually free.
So years later, Turbine finally woke up and decided to make it free to play, with a cash shop of course. They were so close to running another game to its death (*cough*Asheron's Call 2*cough*). I actually liked the game when I played it though, just didn't think it deserved to be as expensive as more 'open' worlds like WoW or EQ2, considering that 80% of the content in DDO is instanced (like Guild Wars).
Venture into dungeons and defeat the evil within!
Being a Dungeon and Dragons fan doesn't necessarily mean that you will like the game. If you enjoy DnD games like Neverwinter Nights however, you will probably like it more. For one, it is action based just like NWN, and no there isn't a 'Parry' and 'Discipline' skill system which ups your character's survival by 1000%.
Having Dungeon and Dragons knowledge naturally gives you an edge over new players in this game. In fact, it is best to do some DnD rules research before making your first character. Builds can be easily screwed if not preplanned well because of the diversity of 3.5e DnD rules. Always try to plan as far as the next 5 levels and realize the role of your class/multiclass. I once met a 8 CHA cleric who can't even turn undead in a level 2 dungeon. I sure as hell not want to meet a 8 CHA sorcerer who can't even cast spells. It can happen.
I'm not going to explain 3.5e DND rules here because that would take a whole day, so I'm just going to touch on the MMO part.
DDO characters do not gain experience points the way other MMOs do. Instead, you will have to do quests for XP. Once the NPC gives you a quest, your character will be required to enter an instance of the dungeon.
Upon pressing the "X" hotkey, you will be flashed something that resembles a scorecard. On it is the calculation of the amount of experience point you have accumulated as you progress through the instance, which you will obtain ONLY when you complete the quest. Killing monsters do NOT give you ANY experience points.
At the top, you will see the base minimal experience points that you can gain from completing the quest. Positive percentage multipliers will be added as you find secret doors, disable traps, kill monsters, destroy breakables, going through the particular dungeon difficulty the first time, and many more. Negative multipliers are gained from leaving the dungeon, being overleveled for the dungeon, etc. Also, finding and completing optional quests gives you straight unmodified experience points.
Breaking things to gain experience...?
The instances are, overall, pretty well designed. It's fun going through the instances the first time, but after awhile, it will start to get old. Grinding usually involves you repeating a really good instance that gives good exp and loot. Since instances are static, skills such as Spot and Listen will be quite useless as everyone already knows where to find a certain hidden door or trap.
When DDO because free to play, many features of the game are locked. You will be required to 'pay' for such features using Favor Points, which can either be obtained by grinding dungeons (ie just playing the game) or paying with money. Er...I'm talking about REAL money here for those who are wondering. Features locked include: playing Drow, 32 point buy, playing Favored Soul, playing Adventure Pack modules, playing Warforged, and more.
The DDO Store
Adventure Packs are a series of quests that brings you through a few instances, before rewarding you with sweet items. From my past experience when I first played DDO (when it was pay to play and everything is open), these quests are so good, you will want to keep doing them over and over again despite it being boring after awhile (aka grinding). I have to admit that running them for the first time is quite an epic experience.
Because of the locked features, there is so much temptation to pay Turbine up front, unlock stuff and play whatever you want. I think Turbine did a good job in that aspect. When I first returned, I had to fight off the urge to play 32-point buy Drow Bard with Tomes. It wasn't easy considering how gimped I felt playing a 28-point buy character when there's a great build up there which I know I would enjoy. When you play MMO, you want the best of the best right? Here, DDO locked it from us. 28-point buy character can still be pretty good for those who are wondering. Besides, no one will know if you are 32-point buy or 28-point buy, so don't worry about this affecting your grouping options.
On to leveling, DDO right now has 20 levels (it was 10 when I first played). 20 levels seemed to be a tad bit little compared to other MMOs which usually have above 50 levels. The bad news is, DDO's 20 levels felt like a normal MMO's 50 levels, so each level felt pretty long, which means you will be stuck with the same spells/BAB/stats/etc for that period. The good news is that each level is split into 5 parts, and getting past each part will gain you Action Points which you can use to buy Enchancements that will improve your character. So, don't worry about losing the motivation to level up. It won't be that bad.
All in all, DDO is a fun MMORPG to play, espacially since it is (can be) completely free. You don't even need to buy the box. However, you are highly recommended to at least know how some DND rules first before attempting your first character.