Body awareness
3rd person games make escort missions easier by allowing you to keep the protected party in view behind your character's back. I guess to compensate for this in first person the developer could use a surround system with speakers positioned behind the gamer.
3rd person games make stealth missions easier by allowing you to keep both your body out-of-view and your opponents body in view.
3rd person camera behind-the-back is pseudo-FP. Interesting to think what Pikmin and Advance Wars would play like with this perspective.
Also:
Enemies begin using tactics which affect the player instead of the characters. At some point, enemies seem to become aware that the player characters are being controlled by an "outside" intelligence, and adjust their strategies accordingly. Rather than attacking the player characters, enemies may instead position themselves to block the player's view.
source: http://www.scp-wiki.net/scp-1633
Of further interest:
Games that let you slide in and out of first person view
Geist - possess multiple fp views by switching out of bodies
GoldenEye 007: 3rd person player intro (camera revolved around character) to permanent first person view
Killer 7 - 3rd person on-rails to first person combat w/ third person counter attacks
Metal Gear Solid: The Twin Snakes - third person primarily to first person combat (?)
Deus Ex: Human Revolutions - first person primarily to third person on stealth (?)
Decreasing game character display visuals
Body appearance is now under less scrutiny due to its lack of prominence in games which lowers the standards for character visual design in the medium.