I don't think Enterprise ignored cannon that much although it did play loose at times much as voyager had before.
Basic canon violations just off the top of my head:
The very 1st episode had Klingons on Earth well before the 1st contact was recorded previously. Contacting Klingons on EARTH rather than in deep space. (It was supposed to be in 2218)
Romulans and various aliens with cloaking devices when it was recorded that Kirk and the Enterprise were the first to encounter them.
Time travel known (the fact of its existance not how to do it) to Earth and the Vulcans at a time it wasn't yet known (again Kirk and the Enterprise).
The Borg on the Enterprise NX 01 would have been in the Federation databanks and that would have been made available to Picard at least after his report of the Borg contact. That would include the knowledge of Cochranes report of an attack from the FUTURE defeated by others from the future. Again that includes knowledge of time travel.
The Idea that contact with the Klingons happened in deep space came from a TNG episode which I believe was also called first contact. They stated that they preferred to contact a race right before it initiated warp travel to avoid the disaster that it had with the Klingons. It did not specify that it happened in space. I'ld say that having the first contact involve a Klingon courier being shot in the chest by a Montana farmer would count as a disastrous first contact. Add to that, the fact that the humans pretty much ignored the Klingon's requests concerning his return ie. pull the plug and send back his body. Also, where in cannon was it stated that first contact with the Klingons was 2218?
While Enterprise dealt with time travel, they made a point that the majority of the scientific community at the time rejected the idea
The cloak in enterprise seemed to be one of many technologies that the romulans were toying with. Early cloaks might have been abandoned by the time of the war. In balance of terror, the primary source of information on Romulan capabilities that Kirk had to work with came from war records.
Voyager states that knowledge of the borg was much more extensive then we were originally led to believe, but it was restricted to the admiralty. Picard would not have had access to the technology during Q-who.
Like I said, Enterprise played loose with cannon, but never exlicitly broke it.