To say that a play is 'open-ended' is rather broader than saying it has a good follow-up. While the existence of a follow-up sequence says that if the opponent treats your play as gote, you will have some specific local possibility left, open-ended implies there might be several further plays left, and their implications are so limited.
Therefore it is quite flattering to call a middle game move open-ended: this is how things should be in the heart of a well-played game. In the endgame one can attempt to quantify what is being said here by a full calculation of miai counting for a play, looking comprehensively into the future (this would be hard for oyose in some cases). In the opening a play that wasn't open-ended wouldn't be played by a good player in normal circumstances, and the point usually wouldn't arise.
In major ko fights winning the ko may well be open-ended in the way of deep implications for the rest of the game.
Some of the almost sente discussion probably relates as well or better to open-ended plays, as to those with definite follow-ups. One should remember that sente and gote are in part to do with the psychological state of the players.
The antonym is dead-end.