What Are the fsx native models differences?

I usually read people posts irritated that it is not an fsx native model and it's just an fs2004 plane with added views and as schnitzel always says "Yada yada yada." so what Do are fsx native models? And how why do people fuss? I'm fine if The gauges work and everything? And how do you make an fsx acceleration model And what are the advantages? More Polygons E.t.c need a thorough Explanation.