When Mary Jane Watson was first introduced by Stan Lee and John Romita, she was a fun, hip, extroverted party girl. She just wanted to have a good time, and wasn't interested in committing to anyone. She was the "bad" girl in contrast to the wholesome good girl, Gwen Stacy.
When Gwen died, Peter Parker and Mary Jane's innocence died with her. Both of them matured, and over time became stronger people. We learned that behind Mary Jane's party girl facade was a woman escaping from her own inner demons. Eventually, she and Peter committed, then broke up, then committed, proposed, broke up, proposed again, were married, sold their marriage to the devil. You get the idea.
Now, let's look at how Mary Jane was adapted across the various adaptations of Spider-Man.
Mary Jane debuted in the 1990's series, in a manner similar to her first appearance in the comics. "Face it tiger, you just hit the jackpot." But, the similarities between her and when Stan and John created her end there. This Mary Jane was the good girl, who wanted to be in a relationship with Peter. Felicia Hardy flirted with Peter, but wouldn't commit to him. Mary Jane was role reversed. She was given Gwen Stacy's personality, while Felicia was the spicier of the two woman. Eventually, Mary Jane even fell off a bridge (and into a dimensional portal) in a battle with the Green Goblin... a fate that belonged to Gwen Stacy (minus the portal). She never returned, but a trend began.
Mary Jane's next significant appearance was in the Spider-Man movies, where she was played by Kirsten Dunst. In this adaption, Mary Jane was the girl Peter loved since the second grade, but from afar. Mary Jane wasn't a party girl, and wanted a stable relationship. First from Harry, then from peter, then from John Jameson, before finally ending up with Peter. But, once again, she was given Gwen Stacy's personality.
Then, when "Spider-Man 3" hit, Gwen Stacy appeared in the movie, and ironically, she had a lot more in common with Stan and John's Mary Jane Watson than the Mary Jane of the movies did. The two of them were role reversed. This is even funnier when you realize that in real life, Kirsten Dunst is blond and Bryce Dallas Howard is a redhead.
While this was all going on, "Ultimate Spider-Man" was on the shelves, and Mary Jane was the shy science geek and book worm, and Gwen Stacy the cool, extrovert. The irony is now so thick, you could eat it with a spoon.
Then "The Spectacular Spider-Man" hit the airwaves, and Mary Jane did not show up until the end of episode six, prompting many to wonder where she was, and almost as many to fear she would not be in the series. Of course, to the fans familiar with the comics, they knew exactly what was going on. Mary Jane debuted with her infamous "Face it tiger, you just hit the jackpot" straight out of Stan and John's comics. But, unlike the 90's cartoon, the similarities did NOT end there for once.
Spectacular's Mary Jane was the party girl who did not want to be tied down with anyone, who went on a date with Peter, and stayed friends with him before briefly dating Flash Thompson and ending it when he began to get too attached. Many fans who were unfamiliar with the Mary Jane of the comic books were not happy about this. In the second season, Mary Jane got involved in a tragic romance with Mark Allen, thinking it would be a casual thing, and then all of a sudden being in a relationship. Unfortunately, the series ended shortly afterward, but it gave us a glimpse of where Mary Jane was going to go.
The next series is "Ultimate Spider-Man" and, if it's anything like it's namesake, we will continue to see the role reversal of Mary Jane Watson in the media perpetuated.