F13 has a really good write up on it, which sums things up nicely:
http://www.f13.net/index2.php?subaction=showfull&id=1135010041&archive=&start_from=&ucat=1&
http://www.f13.net/index2.php?subaction=showfull&id=1135010041&archive=&start_from=&ucat=1&
Not your fathers MMORPG
The most important thing to understand about DDO is Turbine is trying a different take on the genre. People looking strictly for a WoW clone in a D&D flavored wrapper wont find it. For some that may be a good thing, for others not so much. Oh to be sure, it has a lot of the same trappings as your previous favorite graphical-Diku-mud; after all, most fantasy computer role playing games owe a little or very large portion of their structure to the granddaddy of all pen and paper rpgs, Dungeon & Dragons. So it comes as no surprise to find this game with familiar fantasy races, defined character classes, and easily recognizable fantasy monsters, magic items and spells. But, as I have grown fond of saying these last several years, the devils in the implementation.
Not only is combat paced differently, but also general movement is more active. You can jump, swim, climb, tumble, smash crates/barrels, trip, be knocked down, feather fall, and mantle (yes mantle) in this game. Swimming is directly affected by both your swim skill and your encumbrance, so taking off your armor is generally a good idea unless you like drowning (course, this being D&D you can obtain items of Underwater Action). All the movement options can come in real handy when faced with the various obstacles, traps as challenges youll find in various quest dungeons.
If combat pace is the #1 difference, the general advancement facet of the game is difference 1A. Ok, heres the deal. Most mmorpgs are known for their large leveling curves, cynically thought to exist purely for the purposes of customer retention. From EQs 50 levels, to WoWs 60, to SWGs huge skill box grinds, most gamers expect a pretty extensive leveling curve with lots of Ding! Gratz! along the way.
On release DDO will have 10 levels, period.
If you never played D&D before, recognizing a level 8 character as being high level requires an adjustment in thinking. That being said, you wont go from 1-10 in a long weekend. Turbine has taken the standard 3.5 D&D experience curve and multiplied by a factor of 10 due to the nature of the game. So instead of it taking 1000 experience to reach level 2, in DDO is 10,000. To counter act the long time between levels and the corresponding lack of ding moments, Turbine has added 4 Action Points in each level. So, when you earn 1/5 of a level, you gain an action point you can spend at a class trainer to get an improved stat, skill, new ability or feat; in essence, they are mini-levels. You can only have 4 action point upgrades at one time, and you can trade up to more powerful versions of earlier action point abilities. By way of example, my cleric currently has the Dwarvern Faith enhancement (adds spell points), Energy of the Zealot IV (yet more spell points), a +2 Wisdom enhancement (upgraded from an earlier +1), and a Devotion enhancement which makes my level 1 and 2 healing spells 40% more effective.
Now, how exactly to you earn experience in DDO anyway? Only one answer my friends, questing. As of this writing, you earn experience only from completing story-focused quests, and all quests take place in private instances. There are no outdoor wildness areas with respawning mobs to hunt over and over (well Turbine is experimenting with a concept of freeform adventuring areas but its not going to be like hunting through any zone of WoW). Loot is pretty much only received from chests inside quest, quest completions or collection quests. This is another major adjustment in thinking, and coupled with the 1-10 level curve they comprise decision point #2.
So, whats the catch?
Remember when I said you could basically tell people about DDO by what it doesnt have by as much as it does? Well, here the other side. DDO is not a very worldy game at all. The non dungeon areas are basically public spaces for meeting up with other people and getting quests. There is no player economy to speak of (and no easy mechanism for one), no housing, no exploration beyond exploring the quest instances themselves, no pvp, no crafting, no elder game, and no wookie table dancing. In short, its a D&D quest and combat game. Some people have said its not even a real mmorpg at all, just a public hub and instance system like Guild Wars but personally, the semantics dont bother me. I enjoy it for what it does differently than other MMOGs.
Now, there is a concern about longevity. Right now, its projected that DDO will have about 130+ quests in it at launch, which doesnt sound like enough. Even though a DDO quests is typically worth about 10 WoW kill 20 raptor and bring me their teeth and Ill give you another kill x quest style quests, you could easily run out of content in a month or two if youre a heavy player. Turbine has said they are focused on adding more content on an ongoing basis when the game is live, and based on their AC1 track record they may be able to do so, but Im not sure to what degree. I am sure they are planning an expansion, which would raise the level cap as well.
Of course, being in beta, there are still plenty of bugs to fix and tweaks to make, but overall, I think its a fun game and worth checking out as long as you know what you are getting into. Having the right expectations of what DDO both IS and ISNT will go a long way to making your decision to try it or not for you.