Multiclass enhancements

One of the biggest character-design features that separates DDO from PnP D&D is the four "enhancement" slots every character recieves, and which can be changed four times per level, as you gain XP.

The enhancements you are allowed to select are determined either by your race and character (total) level, or by your levels in a particular class. A halfling rogue4/ranger3 could pick enhancements from rogue 4, ranger 3, or halfing 7.

The enhancements give substantial extra power. At low levels, each enhancement is almost as good as 1 normal feat, and they rapidly improve as you advance. By 10th level, one enhancement can be worth 3 or even 4 feats. (To compare the value of a feat versus an enhancement, look at the Weapon Focus Thrown feat versus the Halfling Thrown Weapon Attack enhancement series).

Some of the best enhancements are the ability score improvements, which will eventually give you a +3 to the primary trait of their class (except for barbarians, who get the bonus in consitution instead of strength). Nonhumans also get a boost in one of their race's advantaged ability scores, which makes it additionally important to choose a race matching your class. (Humans, instead, get a lower bonus to any ability score). Those bonuses are separate from the usual ability improvements at 4th and 8th level.

The consequence of the above rules is that multiclassed characters cannot get the same quality of enhancements as a single-class character