One of the great things about The Bourne Supremacy and The Bourne Ultimatum (and to a lesser extent The Bourne Identity) is the way they jump right into the action, with Bourne in motion and ready to confront his enemies. Over the course of three movies, the original trilogy reveals more about Bourne's background, but that's never the main focus of any of the movies, and the details remain minimal. When he says "I remember everything" in Ultimatum, the audience has only seen a small portion of the events that he's recovered. In contrast, Legacy is crammed full of back story, with a narrative that focuses almost entirely on the kind of experiences that Bourne had before we met him in the previous movies. Cross is still in training as an agent of the military-sponsored program known as Outcome; he has all his memories and is very much aware of the program's purpose and structure, even if he's been kept in the dark about some of the details and logistics.
As such, the first hour or so of this overly long movie (at 135 minutes, it's easily the longest in the series) is devoted to getting Cross into a place where he has to go on the run and escape from his superiors, and for an action movie, Legacy contains very little action. There are really only two big action set pieces, one around the middle when Cross rescues scientist Marta Shearing (Rachel Weisz) from a hit squad sent to kill her at her home, the other at the movie's climax, as Cross and Marta are fleeing from the series' latest version of the fellow super-assassin activated to take down the hero. That last sequence, a motorcycle chase through the streets of Manila, is actually quite exciting and well-crafted, and it dials down the ridiculous, over-the-top mayhem of the car chases in the previous two movies. It's one of the strongest action sequences in the whole series, but it comes too little, too late, at nearly two hours into the running time.
Taking a cue from Ultimatum, parts of Legacy take place in between scenes of the previous movie, and Ultimatum supporting players Joan Allen, David Strathairn, Albert Finney and Scott Glenn each show up for a single, presumably contractually obligated scene (only Glenn actually interacts with any of the main characters from this movie). Bourne's name also gets mentioned frequently, and the idea is that because of his efforts at exposing the CIA's secret training programs (Treadstone and Blackbriar), this mostly unrelated military-backed private program must be shut down. Edward Norton plays the latest version of the ruthless bureaucrat determined to stamp out a rogue agent, and aside from being younger than previous versions played by Chris Cooper, Brian Cox, etc., he's pretty much the same character.
It makes sense not to have Cross as a carbon copy of Bourne, but by showing so much of his training and enhancements, the movie basically turns him into a superhero. There's no espionage in this movie, no secret government missions. It's just a rogue experimental subject being hunted down. Weisz gets probably the biggest role for any woman in the entire series, and she mostly holds her own, but Gilroy can never make Cross and Marta's escape feel like it matters even half as much as anything Bourne has done. After the great chase in Manila, the ending is an anticlimax; there's no catharsis or final confrontation, just Cross and Marta slipping away on a boat while all of their evil adversaries appear to remain in power. Just before Jason Bourne was announced, Universal had green-lit another Aaron Cross movie, but even with Bourne's return, it's hard to imagine why anyone would care to see the further adventures of this guy.
My final Bourne rankings (after seeing Jason Bourne):
The Bourne Supremacy
The Bourne Identity
The Bourne Ultimatum
Jason Bourne
The Bourne Legacy