Ok so...
Characters: While the lords are bland and Mary Sues (with Eprahim being debatable due to combat) the rest of the cast is great. Innes, L'Arachel, Lute, Ross, Garcia, Natasha, Gerik, Forde and especially Joshua are some of my favorite characters in the series. The villains range from complex and interesting (Selena, Orson), unique in motivations (Lyon) or just fun to watch (Caellach, Valter).
Gameplay: While the game is easy, it also introduces one of my favorite mechanics in the series: branching promotions. It also has the map system which is tons of fun, and the maps have some good level designs in them (say all you want about Father and Son, to me that chapter is well designed due to the change in strategy).
Story: it may be simple but some darker elements in the plot like Orson's madness and the fall of Jehanna add a nice touch to it (altough FE7 has the better plot).