Something I noticed in general is that people seem to choose characters because they either feel good or they are easy to learn based on the PM character popularity thread. Let's run down the top 10 popular characters list which is here
http://smashboards.com/rankings/project-m/league/teams.
1. Fox: Movement feels great and fun to play as when mastered.
2. Marth: Movement feels great and easy to pick up.
3. Mario: Easy to pick up and fun to play.
4. Link: Easy to pick up and fun to play.
5. Sheik: Easy to pick up and movement feels good.
6. Meta Knight: Movement feels great and fun to play.
7. Ganon: Easy to pick up and fun to play.
8. Wolf: Movement feels great and fun to play.
9. Captain Falcon: Movement is excellent and is the manliest character.
10. Diddy: Easy to pick up and fun to play.
Also most of these characters do extremely well in tournament.