Portkey of the World Cup
The portkey of the World Cup is designed to transport a lot of people; the Weasley family, Harry, Hermione and Diggory (father and son), and we read that two other families live in the sector of this portkey.
10 or more people must touch something like a boot at exactly the same time?
Not very useful, it needs a lot of synchronisation (and, as Obie says, a muggle could touch it).
That explains why this portkey needs to wait, it's programmed for several people at one definite time.
The portkey in the maze
Crouch Junior doesn't know when Harry will find the portkey.
He can destroy the obstacle and use Imperio to make sure others don't reach the center at the same time (Krum knocks out Fleur and attacks Cedric, but Harry stops him), and a portkey with an immediate activation is the best solution.
Voldemort probably planned to send Harry's body back with a magical beast wound (to mask the real cause of death). That explains why the portkey can go back too.
Bonus: Theory I read once
Voldemort planned to come back to Hogwarts with Harry's appearance.
He can spy on Dumbledore and the teachers, recruits students, kill Dumbledore and other enemies and take Hogwarts.
During this time Death Eaters can prepare to take over the Ministry.
Voldemort can come back with Ministry and Hogwarts in his pocket, Dumbledore and Harry dead. I don't think the British wizards would resist much after something like that.
And that explains why the portkey can come back to Hogwarts. (Yes, I like this theory, but I can't remember where I found it)