The main change in plot is focused on China instead of Egypt. You've seen Egypt twice before, why not a another change? But the execution was poor. Everything is tonally different. The way its shot and the editing is hardly consistent with the last two pictures, favoring a lot of cuts and congested camera framing, making action set pieces that seem potentially fun and silly turn incomprehensible and joyless. This film falls into CG syndrome and favors a lot of CGI effects that looks like a video game. Clearly, handing the film off to a director who's had a mediocre-bad track record was bad: this is Rob Cohen ("Fast and Furious", "xXx"); and a bad script written by Alfred Gough and Miles Millar ("Smalville"). Even the characters aren't as fun here. Brendan Fraser seems tired and has lost his comic touch (maybe cause he's ironically older in this film, taking place 13 years after the second). For Evy's role, Rachel Weisz (British) did a phenomenal job portraying a cute, smart and feisty librarian ready for anything in the previous ones; but this time, she is replaced by Maria Bello (American), who turns her character into a soft and fluffy bunny of woman, who is anything and less strong. She looks like Rick's mother than his wife, and her English accent was unconvincing too. The chemistry between Rick and this new "Evy" was poor. Even the brother Jonathan is reduced to bad comic relief, and a grown-up Alex, is played by a big-bottomed guy (Luke Ford) who looks like older than his dad; the saddest thing about both characters, was is that they speak with a fluid American accent when they are supposed to be British. The villain of this sequel is lame, while the previous mummy portrayed by Arnold Vosloo was terrifying, this new "mummy", portrayed by Jet Li isn't intimidating and he had an only ONE fight scene. The only actress who does her best was Michelle Yeoh, whom character serves as a Chinese version of Ardeth Bay.
Something good...a pack of yetis kicking the baddies' butts and celebrating like American football players; and comes the Dragon Emperor, who is really Toho's King Ghidorah. Godzilla didn't come because he is Japanese, although of course, that to America doesn't matter, for this, China and Japan it is the same. What is it trying to be? Fantasy? History? Myth? It ends up being a cloudy haze of a mess.
For the end, after defeating the Emperor, everything is happiness and fortune cookies, and Jonathan says "I wanna go to a place without mummies...Peru, for example". The movie ends with a postscript for a fourth movie that says that years after, mummies were discovered in Peru. Maybe would be Inca mummies.
If we could've just waited for Stephen Sommers, a better script, and have convinced Weisz to actually join the film with the better script (she declined this film because she disagreed with the script used), we could've had a nice return for the characters. Unfortunately, this big, dull, joyless bland return for the franchise may be the last we ever see, and its a bad way to go. Surely isn't terrible? Maybe average, right? I'm surprised to say it really is terrible. Unwatchable? Yes, it is painful to watch as you certainly feel heartbroken and insulted to see that a chance for a return for these likable characters was wasted off on a poorly executed effort like this, and to know there's likely no chance in hell we'll get another attempt to do so ever again.
P.S.: For all the reviewers who said that Maria Bello is a better replacement for Rachel Weisz for Evy's role (especially by that Rachel, who is Weisz's namesake); yeah sure, maybe you say it because Bello is blonde. Folks, when "The Mummy 4" (in my country, Peru) appears, don't wait for Rachel Weisz refusing once more to play Evy, and instead put the all-American Reese Witherspoon (learning Inca culture, martial arts and guns expert, speaking with an unconvincing English accent and voila!) to see if you like her (especially that Rachel, who sure was born in 1976). Bon appetit. But I warn you that if you sin of being open-minded, just go see Power Rangers.