New features of the mod compared with the base game.

Well, where do I start?

The mod introduces a whole array of new features, as you've probably noticed. Aside from the lovely improvements to graphics, several directly alter the game outside of the new units, heroes etc. which are explained elsewhere:
 * Landing craft now have guns on them, so don't try to just rush an enemy landing zone.
 * Some turrets have been altered - AA guns can be manually directed against ground targets for ridiculously OP damage, whilst sensor nodes now reveal a huge portion of the map.
 * Technology now develops differently. All ground units are available from the start with the right buildings, whilst 'space tech' structures must be built to advance in space units. The Empire and Rebellion can transform into the Imperial Remnant and New Republic respectively.
 * The map is now much bigger, with new planets and planetary bonuses. Some planets reuse existing maps whilst others have nice new ones. Also note that some planets start as neutral and so can be captured easily with a single unit.
 * Whilst garrisons in the base game were infinite, in this mod a garrison is deployed twice only. So for any garrisons mentioned, double the figure involved. This means that you can win a battle by destroying all the enemy units, since the buildings will then be defenceless.
 * The hero rescue mechanic. If an insignificant hero dies, they are permanently dead, end of. If they are important (ie. everyone has heard of them) then you should receive a mission shortly after their death, telling you to conquer a certain planet to get them back. No nice timers any more.