Hawkeye was one of the greatest archers but was not so effective because other avengers have superpowers.
Agent Coulson was the one who helped in building the Avengers team but at the time of the fight he was easily killed and it was a big disappointment.
Falcon was one of the important characters in Avengers but was overshadowed by the other big characters and personalities of Marvel.
In Incredible Hulk, Hulk was considered the most powerful and angriest character but in Avengers his powers were not shown as per the comics.
Ultron was Seen In Avengers: Age of Ultron but his powers were not shown the way he talked and could have been in the movie.
The Winter Soldier was seen in Captain America: The Winter Soldier but during the Avengers movie he did not create much impact.
Quicksilver was introduced in Avengers: Age Of Ultron and died in the same movie and his role wasn’t shown as per his powers in the movie.