So many movies could fall into the category of under appreciated because their legacy is dragged down by bad initial critical reviews.
When critics bash movies, however, it is because they just feel disappointed with respect to their expectations. I'm victim to this myself. For example, the reason that I was disappointed in Elizabethtown, was because I liked Cameron Crowe's Almost Famous or Fast Times at Ridgemont High so much and brought those expectations with me into the theater. Looking back, I don't think I gave the movie a fair shot because I brought my expectations with me into the theater and I judged it on a scale of disappointing/living up to my expectations vs objectively good/objectively bad.
This is really blatantly obvious in all kinds of instances when it really is as ethically wrong as movie reviewing gets. Although it's just movie reviewing, most movie critics are really giving skewed and inaccurate grades to things like X-Men 3 or Cars. In the former, they say, "not as good as X-Men 2" and in the latter they say, "not as good as other Pixar films like Toy Story." The critics would still rather see the movie than not see it, and they still would recommend it over other films that they might have given higher ratings too, but the bars get reset by the prequels, and in the interest of giving credit where credit is due, it's a little wrong. You'll see this in tons of instances with sequels and in many cases the sequels are just that bad, so it's hard to differentiate when the critics are just disappointed or it's really bad but it's not hard if you look closely. I have yet to read a review of X-Men 3 where they point to a concrete example of something the film did wrong as a film. I personally find that I tend to like the back ends of trilogies compared to other critics who seem to be unable to accept that the initial joy they have of being exposed to the filmmaker's universe can't be repeated.
Another thing to take into account is that a full-time film reviewer sees 200-300 films from each calender year*. That's why a relatively innocent film that sticks close to genre conventions like A Guy Thing, Hollywood Homicide, The Guardian, Night at the Museum, or Madagascar might make for decent lightweight entertainment for a movie viewer but might make for a critic's worst nightmare after seeing that same storyline 10 times that week. That's why a film like "Children of Men" or "Amilie" which I (as someone who sees about 30 calender films a year) felt were somewhat jumbled up in their plot, would impress a reviewer simply because it stuck out of the pack.
Lastly, sometimes movies can be done in by their own hype. Take the recent release Bobby. Bobby was earlier this year labeled as a front-runner for the oscar after scoring really good reviews at the Toronto Film Festival. The rest of the critics, who as per standard procedure screened the movie a week or two before opening date and reviewed it then, felt it was not as good as the initial hype and proceeded to mention their dissapointment in that respect. Outlets that summarize reviews like imdb.com's studio news or box office mojo as well as outlets like film experience, oscar jam, oscar igloo (outlets that in previewing the oscars try to condense reviews to get an idea of what the "critical success" factor is), etc. picked up on those keywords that summarized that dissapointment and labeled it as "critical failure" disregarding what the reviewers actually thought of the movie. So to review, Group of critics A who were present at Toronto Film Festival liked Bobby. Group of Critics B liked it less than Group A, and expressed their dissapointment with group A's initial opinions. This led people in Group C, critical summarizers, interpreted as a bad film. What would have just happened if the film was seen by Group A and Group B at the same time rather than the emotional roller-coaster of hype and dissapointment? Then Group C would have a more exact representation to draw from, or maybe Group C could be careful to consider those factors and not dismiss the earned merits of a movie.
*Calender year is just a term I've coined (spread it around and make me famous!) which refers to movies you've seen that are released in the year you're watching them. For example, if I had been to the movie theater 3 times this year and seen Little Miss Sunshine, Cars, and Stranger than Fiction and I saw V for Vendetta on DVD, then I had seen four movies this calender year.