Well, I don't really want to seem in any way elitist or arrogant, so I guess I will explain what I don't see in other Yoshi's.
1) Spacing of Yoshi mains, in general, is very poor. They bair through their opponent and -hope- that they arent shield grabbed or punished. Many of their moves miss and its a large amount of waste on their part.
2) They do not change up their styles. People will use only his bair as an approach and forget about other ways to get inside the opponent. This makes it extremely easy to read. How about after a couple bairs, where your opponent learns to shield, you pretend to go for bair, but DJC to egglay instead. This goes for on the edge and off the edge as well. Yoshis are too predictable in their movements.
3) They also have a lack of safety in their moves. I think that Yoshi mains have too much pride and fail to realize their character's limitations when dealing with off the edge as well as other situations. Pride ledge drops and uses 2-3 eggs very far from the edge. The chances of you being gimped in this situation are too great and will happen eventually. Not only that but in almost no situation should you ever use downb in the air, and yet I see many Yoshis use it that way. There is too much lag and not enough knockback to be viable. You will be punished.
4) Standing still when throwing eggs. No matter what "style" a person has, such as lilb, not using ETS in every egg toss situation on the ground isn't a smart idea. Why would you not like to gain that extra amount of spacing? Not only that, but it is very hard to read a yoshi's movements when he is constantly weaving one way and another. Standing still and tossing eggs leaves you open. Marth can just run right under them and punish.
5) As you guessed, knowing a matchup inside and out. With every character, you should have an answer for any situation you are in. The reason why G&W, Marth, and Wolf are 70:30 is because we do not have an answer for their attacks. In fact, the only safe option in all of these situations is pivot grab...nothing else. People don't know this. They need to know what move beats tornado as well as anything else.
6) There is a loss of general mind games and unpredictability. Yoshi mains tend to trade in their mind games so they can try to combo as much as possible. They don't care as long as they connect bair->utilt->uair. They need to back off and realize when the opponent has gained control of his character, these won't always work so you have to read and predict and not get read or predicted.
This isn't very organized, but I hope this is the answer you were looking for. This is my opinion of it.