PoR lays the groundwork for a mixed legacy in Daein with the story arc of Ashnard abandoning Daein and the time spent among the people of Daein.
It lays the groundwork for the corruption of the Begnion state and the historical racial problems in that country through the Begnion arc.
It lays the groundwork for the general Laguz vs. Beorc powder-keg that Tellius is.
It also has Ike's personal story culminating in the ambitious Black Knight duel. Ike and Soren in particular are used particularly well for a Fire Emblem game to comment on the world they live in through their personality. They also do break with FE's past. Ike is not the traditional lord, and FE is still living in his shadow: One of the various things they do with Awakening's avatar and Chrom is allow you to replay Ike/Soren.
Radiant Dawn's basic premise is to take hold of these factors in a cataclysmic conflict and to subvert suppositions about the nature of the medallion. Micaiah's arc is explicitly tied into Ike in Daein arc. Begnion is explicitly acting upon the worst impulses you saw first hand in PoR.
PoR went for a low-key story but an marked increase in character and world work for the series. RD took this world and gave you the biggest FE conflict yet.
My main problems with RD are:
-It has trouble balancing itself as a strategy game and an RPG. The Dawn Brigade is horribly disadvantaged by the structure of the game wrt leveling and being relevant to the end-game. They aren't killed off or anything, but they are mostly just baggage despite being your initial group.
-Micaiah fails to be interesting, imo. She is not characterized or used nearly as well as Ike in PoR despite being in a similarly interesting-for-the-series position.
-I just don't like how they tied up some stories from PoR, personally.
But, it is clearly a very good strategy game imo and also in so many ways what I wish FE was trying to be.
As to the thing about boring worlds--you're getting at the heart of it, imo. IS, in the 3DS games, tried to extricate the games from that stuff and provide a more fluffy, popular product. I just happen to like the low-key, boring, nerdy stuff of Fire Emblem and I also, again, think it is key to making anything note-worthy of the games (as stories). I do think something of value has been lost. You agree that it has been lost, but don't think it is of value.