I started playing DDO only because it is very similar to NWN. I also know some other NWN players who were showing interest in DDO for the same reason. I have compiled the following short list to give NWN veterans some basic guidelines about certain general differences between NWN 1 and DDO.
Classes/Races:
1. Prestige classes are inside main classes. There is at least Arcane Archer and Kensei (Weapon Master).
2. There are no evil alignments but Pale Master exists in the wizard class and Assassin in the Rogue class.
3. Purple Dragon Knight and Dwarven Defender might appear later (?). There are no RDD, blackguard, CoT, harper scout, shadowdancer.
3. Main classes are the same but DDO doesn't have druids (yet).
4. Bard and Ranger have 6 points to spend on skills each level.
5. Monks are quite different. They have Ki power and special combo moves.
6. You cannot save skill points for later level ups. You must use the skill points at once.
7. Races are more-or-less the same. There are no gnomes but there are warforged.
8. Halflings can wield weapons of large size and wield small/medium sized weapons as 1H-weapons.
9. There are no epic levels, feats and spells. Level cap is 20.
10. DDO has a system of enhancements. There is nothing like that in NWN by default but on some persistent worlds there have been distantly similar systems involving custom feats or abilities with astral points.
11. It is possible to reincarnate the character and start playing with it again from level 1.
Bonuses:
1. DDO is based on rules from D&D edition 3.5.
2. Attack bonuses of the same type do not stack with each other. For example, bless and aid spells do not stack because they give bonuses of the same type.
3. Improved Critical and Keen do not stack.
4. There is no Discipline skill or Knockdown and Disarm feats. There is a Balance skill and Trip feat.
5. Properties from items do not stack. If you wear gloves of +4 strength and boots of +4 strength, you will get only +4 strength.
6. There are no horses or Ride skill or mounted combat feats.
7. There are no Divine Might or Divine Shield feats. Clerics and Paladins have something similar to Divine Might as an enhancement. Its bonuses increase at higher levels.
8. Clerics don't have domains and thus they all have the same spell repertoire.
Spells:
1. There are spell points but wizards and divine spellcasters need to prepare their spells beforehand in a tavern.
2. Bards and Sorcerers learn and use their spells in a very similar way as in NWN.
3. There are no familiars, shapeshifting spells or shapeshifting abilities. However, Summon Monster spells do exist.
4. Many spellcasters love to use Firewall, Blade Barrier and Fire Shield spells. These spells are used very similarly to their equivalents in NWN.
5. There is no Time Stop or Great Sanctuary.
6. Cleric buffs are very similar (Shield of Faith, Aid, Bless, Bull Strength etc., Spell Resistance, Freedom of Movement, Protection from Elements).
Items:
1. Many special items become bound to the character or player account and cannot be traded to other people.
2. DDO has more item properties but it doesn't really have any custom special scripted item properties (a talking sword, genie in a bottle, campfire/sleeping roll or a tent etc.).
3. It is not possible to modify and change apperance of separate armor or weapon parts or dye them to different colors.
4. There are various item crafting systems in DDO and they are quite diverse.
PvP/Death/Penalties:
1. You cannot be pkilled or robbed of your posessions. There is no Pickpocket skill. PvP is only in taverns.
2. There is no exp loss or item drop or loss after death.
RP:
1. No deep role-playing or RP actions which influence the world compared to RP servers in NWN (interaction with DMs). There are some guilds who have RP between their members.
2. There is a scripted "dungeon master" who tells you things during adventures.
Maps:
1. In NWN SoU brought us desert maps and winter maps. In DDO you can get that type of "update" by spending turbine points on adventure packs called Demon Sands and Reaver's Refuge. If you want something that looks visually at least distantly similar to Underdark in HotU, perhaps a good idea is to get adventure pack called Gianthold.
2. You cannot own houses, place objects and furniture in them etc.
3. You can summon NPC henchmen and give them orders but you cannot equip or unequip their items and gear.
Game mechanics:
1. DDO has a third dimension. It is possible to jump, fall and swim. The game is closer to first-person shooter games because of this.
2. You can move and attack at the same time without stopping for a second. As a result, spring attack feat makes much more sense in DDO.