IronArcturus: For instance, let's take on the discovery of the Borg. Originally the Federation/Humanity first met the Borg in the TNG episode "Q Who." Q sent the Enterprise-D to meet the Borg to prove a point. However, in ENT they decided to retcon the Borg in the episode "Regeneration." So now the viewer is left to wonder, "How could humanity first meet the Borg twice?"
Actually, that is a rather neat time loop. First Contact leaves a few damaged drones on Earth. In Enterprise, those drones reactivate, assimilate a ship, attempt to contactthe collective. They are stopped, but they managed to send a message to the collective, and that message is what leads to the Borg sending a ship to the Alpha quadrant and the Borg activity (not revealed to be Borg untill Best of Both Worlds) in the episode Neutral Zone, the finale of TNG first season. And all that follows leads to First Contact. Also, the record of the Enterprise NX-01 encounter with the Borg explains why the parents of Seven of Nine would be investigating the Borg years before the events of "Q Who" (presumably those records and combined with what was learned from the El-Aurian survivors from Generations).
Enterprise gets a lot of shit unjustly, and that includes messing with canon. I really don't think there was anything major in Enterprise that would mess up anything we knew from the other series or movies. Quite the contrary, I think they even managed to explain a few thing well, like the "klingon foreheads" isssue.