One the strongest cases one can make for the early Church expecting an imminent return of Christ is to note their use of the word maranatha, which was used as a greeting in those days. When believers gathered or parted, they didn't say "hello" or "goodbye"; they would say "Maranatha!"
I've encountered some writings that say Maranatha is Hebrew and Greek, but it is actually an Aramaic expression. In fact, it is made up of three Aramaic words: Mar, which means "Lord"; ana, which means "our"; and tha, which means "come."
So when you put it together, maranatha means "Our Lord, come." It perfectly conveys the concept that the Lord could come at any moment. Maranatha is used once in the Bible by Paul as part of a curse. In 1 Corinthians 16:22, Paul said, "If any man love not the Lord Jesus Christ, let him be Anathema. Maranatha." The word "anathema" means "banned," so Paul was saying, "Let him be banned from our Lord's coming."
The interesting thing about maranatha is that it comes in the form of a petition. When a Christian in the early Church would make this statement, he was actually petitioning the Lord to come. This obviously implies the belief that it was possible for Jesus to answer the appeal.
If members of the first-century Church believed that certain events needed to take place before the Savior could return, they would have been silly to greet each other with "maranatha." They lived nearly 2,000 years ago, and yet they seem to have had a deeper awareness of imminency than many of today's Christians.