The expansion introduces 6 new possible races in terms of 3 new coloured pieces – beige, grey, and magenta. Each of the 3 new game boards is double sided (as per the original, where one side is Human while the other is an alien race). For the expansion races, there are 3 Magellan races (Watchers or Wardens or something like that), then on the flip side an alien race. I didn’t end up playing one of these races, so I’m not sure exactly the differences, but they seem to incorporate a lot of new concepts. For example, one of the races (I call them Space Pirates, not sure of their real name) have no Dreadnaughts. For every battle token they draw (-1) they gain an economy (because they’re pirates). And they have different exchange rates for different commodities (e.g. 2-1 cash but 3-1 mineral and science).
To incorporate 9 players, rather than having your homeworld in the 2nd ring out from the galactic centre, you have it in the third ring. This makes things pretty interesting, as many home systems will have 3 exits to other tier 3 systems, and one to a tier 2 system. Generally in the game systems closer to the galactic centre tend to be richer in terms of worlds, but also more hazardous in terms of having ancient ships.
There are new discovery tiles. I’m not sure what they all are, but I know there is a shield that also regenerates your hull, and that there’s a tile which gives you a small amount of all 3 resources (as compared to the normal +8 cash, +6 minerals or +5 science tiles, it provides something like +2 or +3 to each).
There are new unique technologies. These may only be researched by the first race to purchase them. Their costs seem to fit between the existing research spectrum. The normal game techs cost 2/2, 4/3, 6/4, 8/5, 10/6, 12/6, 14/7, and 16/8, while the expansions cost something like 11/7 or 15/10. The one I remember (that I bought) was a 15/10 power source 12. Other stuff includes intelligent hull (+1 hull and +1 to hit) and point defence systems (allow you a chance to shoot down incoming missiles). These technologies can be placed on any of the three trees.
There are new things called Developments you can buy (or maybe build?) which I didn’t quite comprehend. Some sort of artefact thing which gives you additional VP for each artefact diamond you control. Stuff that lets you turn 5 of 2 types of resource into 12 of the other.
There are new ways to transport between systems – Warp Portal icons, which allow transport to any other Warp Portal icon.
Then there are new types of tiles – Ancient Homeworlds, which I believe produce ancient ships and send them out into neighbouring hexes (I never encountered one, so not too sure how it works).
The Galactic Centre Defence System has been upgraded to a tougher fight.
There are also some changes in gameplay. To stop the game stagnating, there are primary and secondary player markers which go on alternate sides of the table – so 2 people are taking their turn at the same time. Only the primary player may research, though. The first player to pass still gets to go first in the next turn, but the second player to pass gets to determine the direction of play for the next turn (clockwise or anti-clockwise).
So all in all – a lot to absorb. As you no doubt gathered from my summary above, I certainly didn’t manage to absorb it all (I had to use the internet while writing this to work out some of the things). My first reaction is that I don’t really like it. Part of the elegance of Eclipse was its simplicity, and the expansion has done away with that.