Star Trek is full of inconsistencies, its always has been. Its the product of a geberational TV show that has had many different writers and producers, as well as a plethora of fan fiction and fan created material that becomes obsolete when 'new' cannon material is produced. Some of the best Trek novels, such as "Federation", were exceptional (sadly, these are exceptional also in that many Trek novels are little more than pulp fiction), and described many events that were not seen in movies or the TV series. In particular, Federation describes the birth of warp flight and the story of Zephram Cochrane, which all became obsolete when First Contact was written.
Unfortunately, Berman and Braga took this level of inconsistency to a new height. I never actually watched Enterprise on TV, and read much about the croticisms people were making of it. However, all 4 seasons are now released on DVD (the last of which was released here in Australia a few weeks ago, thus completing my Star Trek DVD collection - yes, thats right! I have the entire Star Trek collection on DVD!!) and I've been watching the entire series from start to end (only up to season 3 at the moment). While I am, in fact, enjoying the series a great deal, there are a few inconsistencies which I am finding to be very annoying. The episode that has the Ferengi in it, for example. While I actually liked the episode, my impression was that it was an attempt to bring back some of the old style TOS humour that has been sorely lacking from Trek for awhile, using the Ferengi in the episode was reallt just a bit too incongrous for my tastes. For example, if it was possible to stumble across the Ferengi in the 22nd century, then certainly by the 23rd, there would have been more such incidents, let alone the 24th where the Ferengi were initially described as a previously unknown race with very little contact.
The next inconsistency which I found to be just a bit too blatant and entirely uneccessary was discovering photon torpedo launchers on a Klingon ship. In the Original series, the script writer's guide descibed the Klingons and their technology for use by script writers of the show, and the main weapon used by Klngons was called a "Sonic Disruptor pistol". This was also extrapolated later to onboard ship mounted versions. The first time we ever saw Klingons actually use Photon Torpedos was in Star Trek the Motion Picture, the K'Tinga class which had fore and aft photon torpedo tubes. Yet in STIII, a Klingon Bird of Prey is once again armed with Disriptors, as both Klingons and Romulans are in TNG. The K'Tinga class, then, seens to be an aberation, and conjecturely we can allow this inconsitency by assuming that after contact with the Federation for what would have been quite some time by then, that Federation technology had been stolen or copied, just as the Klingons and Romulans had traded technology giving the Klingons a cloaking device. According to Enterprise, however, it was the Federation who copied the Photon technology from the Klingons (Archer makes a comment that he has never heard of Photon Torpedoes when he discovers them on the wrecked Klingon vessel) and then later, we see the Enterprise being refitted with them.
My personal opinion, is that both Braga and Berman had become accustomed to the freedom of creativity they had had with TNG, DS9 and Voyager, the latter in particular. Voyager, out in the Delta Quadrant, was far enough removed from anything previously seen in Trek, that just about anything could be created and it would be believable. Yet, when creating a TV show that is based on historical content, such leverege of artistic license becomes severely curtailed. While both Braga and Berman discuss this in the bonus material interviews on the DVDs, claiming that they wanted to stick with the Original feel of TOS as much as possible, they achieved this only to varying degrees. The set of Enterprise, the hairstyles, and even thhe uniforms do seem to have qualities where this is apparent, nevertheless, there are other areas where consistency was completely ignored. The conspt of Enterprise was an excellent one, however, in my opinion, Braga and Berman were still trying to make it look like a TNG series, despite their comments to the contrary. I attribute this to the pursuit of ratings, since TNG was very successful and also to Paramount and its favouring TNG over TOS.