Yoshi has very few saving qualities, but some of his strengths include his unique abilities.
If Yoshi is positioned right on the edge of a map, he can use his B attack to turn opponents into an egg, and, if executed correctly, the opponent will fall to his death before he can escape the egg.
Yoshi's (B + Down) attack is unique in that it creates shrapnel damage upon landing, making an otherwise easy grab for opponents more difficult.
When executed properly, Yoshi's egg-launching attack can be incredibly annoying and even effective as an edge-guarding technique. The eggs can be employed much the same as other players' aerial back kicks, preventing a player from regaining his second jump, and forcing him to use his third jump until he can't make it back onto the map.
Yoshi's tailwhip attack, smash attacks, and dash attack are incredibly slow and normally can be easily avoided. When Yoshi's smash attacks connect, however, they are pretty powerful and can send players quite a ways.
Yoshi's most powerful move is probably his Flutter Kick (A + Down), a move that can deal opponents about 40% damage if the entire attack connects. The move doesn't propel opponents downward like Kirby or Fox's Flutter Kick, however, making it less effective.
His "throw" attack, executed by extending his tongue and sucking in unwary opponents, is also incredibly slow an easily avoided. The throw is, however, one of the more powerful throws in the game.