Some of the biggest improvements in TI4 are actually things that were left out. A great number of game options from TI3 and its expansions did not make it into TI4 and in my opinion this is one of the best moves FFG made.
Leaders for instance were left out, this game option gave each race three leaders that had various special abilities. Leaders were one of the weakest game options in TI3, they added very little to the fun of the game while adding a great deal to the rules weight and cognitive load on the players. Even seasoned players have a hard time remembering each ability of each different leader type. If you played with this option you inevitably spent some part of the game looking up all the different abilities each leader had multiple times. The leaders relative power level was enormous, it felt pretty bad being stuck with lame generals while other races got diplomats and agents.
Representatives are gone. These guys were added in Shards of the Throne as a way to make the political phase more interesting. While they did give players more to do during the political phase it often ended up seeming like a whole lof of arm waving for not much added fun and it increased greatly the playtime of a notoriously long game. In TI4 you can get a bit of the representative feel from a series of action cards called “riders” that allow you to abstain from a vote while predicting its outcome and being rewarded for it with a bonus. I think the riders are a great compromise and distill the game concepts introduced by the representatives to remain while not adding a lot of mechanical weight to the game.
Shock troops have been left out and good riddance. They added very little to the game and remembering to spawn them on rolls of 10 while also killing them first in combat was a nuisance.
Ion storms were left out, mostly because they weren’t unique enough, They were essentially just more nebulas. I’m happy with this, now I don’t have to remember the difference between the two.
Facilities are gone, they were mostly there to give players in poor segments of the map a non combat option for gaining stats. Again these were not worth the rules weight and I’m glad they are gone.
Mechanized units are gone, Woohoo!! These things were a rules nightmare, this mostly came from the fact that there were immune to most things that affect ground forces. 528
Having them in the base set would have been an opportunity to make them work correctly from the start but I think the designers didn’t even want to bother, again they didn’t add much.
Trade stations and other refresh abilities are gone, they were complicated and mostly pointless. I was not surprised to see these gone. Perhaps we will return to mirage or hopes end in an expansion with a better mechanic.
Transfer action got the boot. This one I’m mixed on. There is a need for players to “rearrange” but the transfer action was a rarely used and complicated way to accomplish it, ultimately I think the game is better off without it but I’m not sure.
Artifacts are no longer in the game. I think they could have left these in, I’ve always liked them and think they add more incentive to combat. That being said it isn’t tragic that they aren’t in the base game, I expect they will return.
Distant suns/final frontiers are gone, removing the much maligned exploration element of TI. I think distant suns in TI3 were badly designed and way too swingy, my group mostly didn’t play with them. Final frontiers however we almost always play with, it makes empty space interesting and gives an incentive for activating an empty system. I understand why FFG left out both options but again I wouldn’t be surprised if they make a triumphant return in an x-pac after a serious rebalance.
Mercenaries did not return for TI4, to be honest I’m not at all surprised. Mercs had a cool feeling to them and added some characters to the game, but they felt like they weren’t playtested very much. They interacted in strange ways with other mechanics and their relative power was off the charts, that combined with the awful trade III strategy card that made them work lead to my playgroup abandoning them fairly quickly.
The friendly fire action card is no more, this card was introduced in shattered empire alongside automated defense turrets, many players of the base TI3 game complained of the dominance of fighter fleets and so FFG added two very powerful nerfs to fighters. In my opinion this was an over nerf, fighters went from being a dominant strategy to being a liability with two swings of the nerf bat. 528