I feel that's the case because I haven't played FE6 to know if that makes the plot any better. And I want to believe there is actually some substance to it compared to what I saw in my two playthroughs of it several months back.
Then this is an assumption with almost no grounding.
FE6 actually receives VERY few references in 7:
Returning characters: Bartre, Karla (referenced in 6), Karel, Marcus, Hector, Eliwood, Merlinus, Canas (referenced in 6 through Lugh/Ray/Niime).
The only characters 7 brings up that really matter to 6 are Roy, Lilina, and Zephiel, all of whom are introduced solely on plot. Everyone else (mentioned above) stand completely on their own with no prior knowledge needed.
Almost no plot is shared between 6 and 7 save for Bern being dickheads and invading ****, and the background "War of the Dragons" stuff that also serves as the grounding for the Divine Weapons (Durandal, Armads, etc).
7 is a PREQUEL, so everything stands pretty much entirely on its own, as nothing in 6 has HAPPENED yet.
Maps that are mostly mountains and rivers so that most units can't move,
There are not that many of these, and they're there to break up the monotony of **** like Awakening's "here's a rectangle" maps, and also provides balance between the different unit types. Cavaliers would be totally busted if they never had to worry about terrain. Magic users get advantages in Deserts, etc etc.
moving bridges that caused me to require another 30 turns on a map while I waited for Hector to stop being suspended in space
Okay, I'll admit 26x is awful, I'd forgotten about that one.
a map that requires a key that doesn't give you one?
Which is this?
I also didn't like how there seemed to be something that resembled a convoy system through Merlinus, but it just flat out didn't exist.
Totally false. Merlinus does exactly what you think he does. Did you fail to recruit him?
How wasn't the final boss bad? Having played the Tellius saga and Shadow Dragon, I've beaten plenty of fire dragons as regular enemies. Now I am supposed to see this generic nameless fire dragon as something impressive? Not particularly, no. Not at all, actually. If it ended at Nergal I would have felt a lot better about it. But no.
How does that make it BAD? I'm going to attribute this at least partially to chronology, as FE7 was the first English (and also my first), so I can see where you're coming from, but perhaps you missed how the entire point of the plot was to PREVENT Dragons from coming back? From the other side of this argument I could complain that Tellius/SD trivialized Dragons by having them all over the place.
Nearly every character was just extremely flat and uninteresting, or was flat out unlikeable. The only exception I can think of this was Nino.
This is entirely opinion, but I consider the following characters to be plenty interesting, as far as one can expect from a game with 40+ characters:
Rebecca, Dart, Will, Hector, Oswin, Matthew, Priscilla, Sain, Florina, Canas, Legault, Heath, Pent, Karel, Nino, Renault.
There are plenty of others I like, but will admit are relatively tropey. (so are some of the above) But Fire Emblem in GENERAL has this problem. How much did you read of the support conversations?