10 Worst Ever Avengers
4. Mister Fantastic
The two other members who joined the Avengers in #300 along with Gilgamesh were Mister Fantastic and the Invisible Woman.
The plot of the issue revolved around their son Franklin being kidnapped. Reed and Sue had both retired from the Fantastic Four at the time and so ended up banding together with a group of other heroes to save him. Afterwards, Captain America made them all Avengers.
Right from the start, they didn't really fit on the team, especially Reed. The problem stemmed from the fact that, as the leader of the F4, he was used to giving orders rather than taking them. This led to him frequently clashing with Captain America and often inadvertently trying to take control of the team during missions.
It was also pretty weird from a readers perspective as well. Mister Fantastic was less of a superhero and more of a scientist and adventurer and the Fantastic Four were more of a family than a conventional superhero group. On a team of conventional heroes like The Avengers, Reed Richards stuck out like a sore (and incredibly bendy) thumb.
Reed and Sue ended up having one of the shortest ever tenures on the Avengers. In #305, Cap reorganised membership into a mission-by-mission basis and they were both put on the reserves list. Reed and Sue have helped The Avengers out from time to time, but never fully rejoined.