"Balance of Terror"
Kirk : "Yes, well the question remains gentleman; can we engage them with a reasonable possibility of victory?"
Scotty : "No question. Their power is simple impulse."
Kirk : "Meaning we can outrun them."
In "Star Trek: Insurrection", Dougherty said the following: "On Earth, petroleum once turned petty thugs into world leaders, warp drive helped to form a bunch of Romulan thugs into an empire. We can handle the Son'a, I'm not worried about them." Picard replied: "Somebody probably said the same thing about the Romulans a century ago." This seems to indicate that Romulans didn't have warp drive prior to 2275.
-Taken from http://www.ex-astris-scientia.org/inconsistencies2a.htm
Have you guys thought of why there was a mine field? I think that there was a mine field because their ships weren't as advance. They needed the mine field to get an advantage. They also had it to set a border for what they call an Empire. The other Solar System surrounding Romulus was probably uninhabited, and they set them for colonization. The cloaking technology was not developed yet also so it was an error in the producer's part.
I think the Romulans in Enterprises' time did not have warp, but something similar to it. It would probably explain why their form of warp is powered by an artificial quantum singularity (miniature black hole). Because they travel in sunlight speed they would encounter time dilations, and as a result would have to study a way to reduce such affect. They would possibly figure out some principles of black holes that would help them gain warp. After all black holes have a high gravitational pull, and would slow down time theoretically. As result they probably gain warp that way.
Also if the Romulans had warp, then they would have won the Earth Romulan war. Earth only has a few warp capable ships, and right now only one warp 5 ship. By the time of the war, and at the rate of construction they probably would have at most 25 ships that is as fast as Enterprise, and a tone of slow ships. Remember the second warp 5 ship is not finished yet and it's already three years after the launch of Enterprise. If they do mass production, then they would have to have multiple shipyards and that would have to take time to construct them. This is if we assume that the warp 5 complex is the only one of its kind, and all other shipyards in the Federation were built for the lesser warp ships. The third warp 5 ship is probably launched a year after the second one is produced, which might be this upcoming 4th season. This is because the process of construction would probably approach a production line speed. Mass production of this type of class would probably begin right after the third ship. Either way when mass production starts, about 5 would probably be produced a year, the process would have begun five years after the launch of Enterprise. This leaves four more years, and five ships per year would put it over 20 ships in Starfleet at the start of the war.
If the Romulans had a major Empire then Earth would have been dust, they probably have a tone of ships and that's why the war was so bloody. Earth probably also rush productions of lesser quality ships in order to counter the Romulans, and probably relied on nuclear weapons, rather than anti-matter because producing anti-matter weapons were probably a rarity at that time. Earth also probably built a tone of ships like the J-class and Y-class ships and fitted shuttles with weapons. Remember the Romulans probably have hundreds of ships, and Earth only a few. Even with warp capability a ton of Romulan ships could just barrage Earth's fleet and severally cripple it. Warp means out run, but the Romulans probably relied on outgunned. If Enterprise could only fire 2 torpedoes every other second, then that would be very slow compared to 5 ships firing torpedoes at the same rate. Imagine Forest Gump with a revolver and a crowd of people throwing rocks at him, even though they might have only hit him several times, and he would have only shot a few people, he would have been hurt, and heal time would take a while. Because there is only a hand full of people like Forest they would have to run a lot with the gun, and sometimes ignore heal time, eventually the damage would build up and then death.
To sum it all up
Romulans = no warp but LOTS of tough ships
Earth = very little advance ships and lots of non-advance ships used as frigates.
Romulans = probably well armed
Earth = ships armed sparingly, and Enterprise lacked phase cannons until they decided they needed it.
To me this is a logical explanition for the reason why the Romuans shouldn't have warp right now. If they did Earth would have been doomed, and done even try the better crew against overwhelming odds, because starfleet's new ships wouldn't have better crews, it would be their first employments.
Now let's point out other continuity mistakes in Enterprise. After all this series was created to bring in "new" audience, and as a result follows a different style than the others, which alienates the devoted trekies. This show is turning into something similar to Episode 1 and 2 of Star Wars. Heavy of the special effects, lackluster on the storyline and plot, and actually worse in some ways when compared to Star Wars, in the sense that it does not answer the questions of the past, and reveal new stuff. Instead it just destroys what we trekies have believed in.