Honestly, Turn Based games are most engaging when the results of battle matter a whole heck of a lot in terms of going through a dungeon. Or when there's multiple challenges that rise to the amount of power the player can acquire (or limits power acquisition in some way).

They should also preferably be snappy with regards to how long battle animations take. In the late PS1 and PS2 lifespans, a lot of turn based RPGs really kicked up the graphics on making attacks huge and cinematic. While very cool looking the first time, do you really want to spend a minute or two every time you summon an Eidolon?

With regards to this, I really like how Xenogears handled battles. You could grind your character's levels all you wanted, but if you didn't master Deathblows, then the set power that Gears were allowed to obtain couldn't be fully exercised. All of the deathblows were cool with powerful collision sounds and effects, and played out within a few seconds. And when you built AP to do up to 7 Deathblows in a row, they played out completely but automatically fast forwarded.