Can anyone tell me if there are major differences between basic impetus and the full rules?

the first difference is that you have bigger armies. How large depend on how you spend your points.
The standard level is 500 points and 3 Commands. To give a rough idea on average you could field an army of 25-30 bases.

You spend points also for the quality of the generals.

The second thing you will notice is a new value among the troops type. This new value is Discipline.
You will have to make a discipline test in some cases. One of this is to recover from Disorder.

In Impetus troops have more tactical options. In other words it is a more articulated game.

I make some of examples.

In Basic you can perform some manouvre in the enemy Unit's Zone of Control. Well in Impetus you can but at your own risk as the enemy Unit will react!!

You can also decide to put in Opportunity a Unit or a Group by passing a Discipline Test. Once in Opportunity (a sort of defensive order) you can act during enemy phase.

Other features are the possibility to evade, countercharge, charge by opportunity etc
Impetuous troops are also more defined.

Thanks. That makes it clearer.