I didn't like the lack of front line ability in D:AO. That is a warrior had no real means or method to protect a caster or archiver from an attacking warrior. There was no method to establish a line or area of control.
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I do not remember this being an issue in older games like ice wind-dale and BG 2; but maybe it was and I just didn't notice ?
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I really liked the presentation in D:AO (story telling/cut scene - which is not quite the same as the story itself).
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Also many of the dungeon/maps I felt were really simplistic/poor. I never felt like it was an epic adventure of exploration.
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So to sum it up I dislike D:AO combat mechanics and maps.