As a DM:
Strengths: Rules smithing, memorable NPC's, world building, emmersion, detailed descriptions, 'plot twists', ruthless 'rat #$@&! DM' tactics, game management
Weaknesses: 'plot hooks', extemporaneous play, waste alot of development time on simulationist details that rarely impact play, ruthless 'rat #@$%! DM' tactics, detailed descriptions, strong NPC's tend to relagate PC's to roles of observers particularly if the player is inexperienced or timid
As a player:
Strengths: Respectful to the DM, strong memorable characterization, strong dungeoneering skills, not a power gamer, not a rules lawyer
Weaknesses: Poor min/maxer, secretly tend to think I can do a better job than the DM, get bored easily with the less frenetic pace of play as a player, when bored tend to do silly plot derailing things to amuse myself