It's free to play...there is no con to at least trying it.
Seriously though, I haven't gotten very far into it yet, but these are some of the things I've seen myself, and read from others in the forums.
****Pros****:
Free to play - About 1/3 of the game is immediately free and open (basically all of what the original game had at release 3 years ago).
The remaining 2/3 of the game can be unlocked for free just by playing the game and earning free store points (which can then be used to buy the remaining locked content).
Several payment options to fit your preferred method. Free (as mentioned above), Premium (same access as free but you can pay in small chunks to immediately and permanently buy access to the content you want), and finally VIP (the $15/mo subscription that unlocks all areas/quests and most races/classes for each month you're subscribed with a few extra goodies thrown in).
Generally a very helpful, mature, and D&D experienced community.
Great graphics (supports dx10 and being worked on already to support dx11 when it's out)
Unique quests - Unlike most mmo's, there are few "kill x of y" and messenger quests, instead quests are designed to complete certain objectives. Many quests and dungeons have actual traps and puzzles of various kinds.
Loot/rewards/xp - Trash mobs do not drop loot. Instead, most loot comes from chests scattered throughout the quest areas. XP is tallied up at the end of the quest for everything that was accomplished. This allows groups to complete the quest objectives in the ways that suit their classes. Each member of the group gets their own loot from the chest, eliminating the need for rolling for loot or loot ninjas. You CAN however assign any loot you don't want to another player while it's still in the chest and unbound. Other forms of loot are found in brake-able jars and crates scattered around.
The game was made for grouping, but with differing difficulty settings on quests, and the newly added scaleable dungeons, it is a lot more solo-friendly if that is your style. NPC hirelings can also be bought to quest alongside you if needed.
****Pro or Con, depending on your preferences****:
Unique combat style - Unlike most mmo's, you don't sit in one spot and spam/cycle through skill hotkeys. Instead, combat is very action/twitch oriented to take advantage of terrain and postitioning. Many spells and arrows can be dodged if you see them coming in time. Some players don't care for this twitch style gaming.
Lots of content designed for each class specifically. Traps that can only be found and disarmed by rogues with the appropriate skills, locked chests/doors that can only be opened by rogues or casters with "knock" spell, some stuck doors requiring high str characters to open, etc...making each class useful in it's own way. Unfortunately this also means that if you solo or don't have the right class in your group there may be some side quests and chests that you can't access and will have to skip.
D&D online is based on the Eberron setting (which is a newer and more unfamiliar setting) and NOT on the more familiar Forgotten Realms and other more popular settings. This is what WotC wanted when DDO was being made, so this is what Turbine had to go with. This also allows Turbine to be more creative and original in what they design since the setting hasn't already been pre-built by years of books, comic books, and official campaign packs.
DDO is very heavily instanced. All your quests will take place in areas that are open to only you and your group. You will not see other players running through the wilderness or dungeon where you're at. This also means you will not have to compete over mobs and chests. The towns and quest hubs are where you will be interacting with other players.
Character creation is detailed and complicated (easier if you already have a good D&D background). Using a pre-made character path can help out, but if custom making your own it is very easy to accidentally gimp yourself or lock yourself out of abilites you may want later on. This also means there is no cookie-cutter build and you really can design your character however you want it...even being able to multi-class.
****Cons****:
If you play for free and you want to unlock other quests/classes without paying, earning the free store points in game will take a lot of time and will require repeating quests over and over on several different characters.
Free players are limited to 2 characters per server.
Guild features are lacking in a lot of ways. There is no guild housing or storage. Basically the only benefit to being in a guild is the social one of having a steady group of friends to quest with.
Free players have extra limitations placed on them, many of them designed to prevent gold famers from making throw away accounts and ruining the game with spam messaging and gold trading. Free players have some chat, auction, and mail restrictions as well as having a cap on how much gold they can carry. The good news is that a free player can remove these limitations with one purchase of the cheapest Point package on the DDO store. Doing this will give Premium status, removing the limitations and giving you 2 extra character slots per server.
This was a long post, but hopefully informative.
Good luck.
