I think that about sums up Gundam SEED Destiny. It had the potential to be one of the best Gundam series ever. For whatever reason, that never materialized.
Let's start with the mid-series shift from Shinn to Kira as the main character. While I have heard theories that the shift was intended all along, I have serious doubts, mostly due to much of the early plot threads being abandoned. The Rey vs. Neo that was teased at early? Never materialized. The Zala Loyalist faction that dropped Junius 7? Never even mentioned after that. Possible character development for Auel when we start learning about his mother? He died a couple episodes later, ending that forever. Heine was brought in, died, and served no purpose in storytelling. We never even found out what Sting's block word was.
Another problem is the lack of character development for the SEED cast. The mid-season shift focused the story on them, but were their views ever challenged? Did they ever make mistakes that the show acknowledged as mistakes? Did anybody ever call them out on the shit that they pulled? The answer to all of those questions is no. Later Destiny shifted the focus to them, but they received zero character development, positive or negative.
Another point that Destiny could have been great on but dropped the ball was arguments. And by arguments, I mean the lack thereof. There were very rarely any actual arguments in the series, instead what would happen is one side would say something, and the other would stand there with their jaw hanging open, their minds utterly blown. This becomes especially irritating when there are valid counterpoints to be made. It happened to Cagalli, it happened to Shinn, it happened to Athrun. Plain bad writing.
While I do not subscribe to the theory that Kira and Lacus are self-inserts, I can see where those who do believe it are coming from. Lacus is always portrayed as right, and the entire series runs on “That which aids Lacus Clyne is good, that which hinders her is bad”, regardless of how much, or little sense it makes. Probably the best example is Mu La Flaga, who upon switching sides never gets anything even resembling comeuppance for his part in wiping three cities off the map, brainwashing child soldiers, or kicking off the entire war.
There's too much I would like to say that I can’t in 400 words, but “Wasted Potential” sums things up.