Making a movie prequel can be a very tricky thing to do. Occasionally, they can turn out to be pretty decent like X-Men: First Class and Monsters University, but making a prequel to a more established and popular film franchise (especially one that isn't a comedy) is a process that can be more than a little bit problematic. Issues that are endemic to prequels mainly relate to the restrictions of having to fit the film in with a franchise's established canon, and audiences' appreciation of the series that prequel built on potentially colouring their opinion of it. These are four flaws faced by movie prequels...
4. Continuity Is An Almost No-Win Situation
Dealing with continuity in a prequel is something that's very difficult to get right. If you incorporate too little of the original film(s), it's likely that both critics and its fans will criticise you for not acknowledging it enough and not tying the original and the prequel together well enough. And if you do a lot more, it's also likely that they'll criticise you for relying too much on the original film(s) and not being creative enough. You can't do right for doing wrong. Apart from minor elements like the mission the film is based around being financed by the Weyland Corporation, the inclusion of Facehuggers, and the appearance of a proto-Xenmorph at the end, Ridley Scott's Alien prequel Prometheus largely stands on its own two feet and isn't reliant on the mythos of the Alien series to tell its story. So because of this and it leading into an upcoming second prequel with none of the big questions from Alien being answered as well as several of its own mysteries being unsolved, some Alien fans were displeased with the perceived lack of connection to the original films. For an example of the exact opposite, look no further than the Star Wars Prequels, which were filled to bursting with characters, settings, and stylistic elements lifted from the Original Trilogy as well as a plot that tied directly into it. If you watch RedLetterMedia's feature-length review of Attack Of The Clones, you'll see that one of the things that they bring up and discuss at length is how much of the film is "ripped off" from The Empire Strikes Back, including certain shots and vaguely similar concepts and plot points. Many Star Wars fans objected to the inclusion of so much material from the Original Trilogy (especially Boba Fett's extended cameo) which they saw as largely unnecessary and it became one of the reasons that the Prequels were so poorly received. It's hard to find the middle ground between linking a prequel to the original and relying too much on it, especially since what is an appropriate level of continuity to include can vary on a film by film basis and a lot of critics and fans have different views on the subject. X-Men: First Class is a film that feels like it managed to strike a good balance between the two extremes though, despite it including a large number of retcons that give it the feel of a semi-reboot at times.