I did not mean dialog system, inventory, particles etc , these are what I mean for essential features
A game engine is of great help to accomplish such functions
I was speking about the game play

enemies can either pop up from the bushes or they can plan a strategy

In the former case you have a boring game even with a fantastic graphics in the latter case you have an exciting game even with poor graphics

The differance between the two is some thousand lines code
Little to do with your favourite engine