This is how I feel about it. I tried to get into Shadow Dragon, but everything just feels so bland and there's a lot of stupid stuff like gaiden chapters and killing off units to get other ones. I haven't played the game in years besides some "I'll try again" moments to then just put down the game. FE12 has supports, better growths, good difficulty options and curves, and gameplay.
Enemies in FE11 from my memory are kinda just placed in spots with no plans of their own. In FE12, enemies are placed in ways that they all attack one spot maybe 3 or even 4 times. Enemy formations are a bit scary to break sometimes on harder difficulties.