One thing that always bugged me about Voyager is that several of the characters didn't seem to know about the Dominion.
EMH 2: The Romulans haven't got involved in our conflict with the Dominion
EMH 1: The who?
You'd think that the EMH would be programmed with knowledge about the Federation's newest threat (VOY season 1 occurs during DS9 season three, when the Dominion threat was real, and known about). Particularly since the Voyager holodeck had a shuttle training programme with Jem 'Hadar fighters as the opponents.
Chakotay: The Cardassians had help from a new ally (when describing the massacre of the Maquis)
We did get a few crossovers between TNG and DS9, and TNG and VOY, but not many between DS9 and VOY (except Doc Zimmerman, mirror Tuvok, and Quark showing up in Caretaker)