The movie is in essence a retelling of the 26 episode series. The story is focused around Hitomi, who is transported from her own world after an encounter with Van and the Dragon Armor to Gaea. Hitomi turns out to be the deciding factor in a decisive battle between Prince Van and his brother Lord Falken.
There are some notable differences between the movie and the series. Most have to do with character development and tone. Since the movie is trying to capture the characters from a 26 episode series in an hour and a half, there isn't a great deal of time to explain the complexity behind each characters story. Because of this, much of the development is focused around Hitomi and Van and their relationships with the other characters. In the movie, Van is portrayed as a much more lone and aggressive individual. Hitomi is very depressed and lone as well. Much of the tone set by this movie is one of seriousness and to a degree sadness. There is a fair bit of action, but not the heart pumping action that you would see in a show like Dragon Ball Z or Naruto. The overall effect is a feeling of violence and hatred.
The animation style appears to be geared towards an older audience as well. There is a lot of use of dark colors. The characters also look older and have a greater degree of seriousness in their design. Escaflowne has always been known for its use of orchestral soundtracks through the series. The movie is no exception. Having listened to both soundtracks, I would have to say that the soundtrack used for the movie is much more depressing in the emotions it evokes. The songs have a very sad sound to them and express emotions of pain.