What's wrong with discovery? Perhaps it was crap if you played multiplayer, I wouldn't know. But for singleplayer, all it did was add to the experience for me.
Poor balancing. The bombers are just as small as fighters barring two exceptions, and turn only somewhat slower than they do. It basically means that bombers can ignore escort fighters while pounding on a capital.
Many, many instances of poor balancing that aren't as large in scale that I won't go into.
Pirates having battleships. I mean really, the Blood Dragons are a small, SMALL organization who somehow managed to find battleships out of thin air. The Corsairs and Outcasts may have planets but lack the industry, among other things, to produce and field capital vessels.
Numerous stupidly empty systems that serve little to no purpose.
Horrible models with even worse texturing.
The story bits added are absolute garbage too. Kusari and Bretonia only just going to war over a few system that have been around for ages and have relatively little in them, Liberty and Rheinland going to war because Liberty wants a border system so that they can build prisons there (lolwut, they already own plenty of systems for that), the Nomads still being around despite being removed in the singleplayer (retcon lol), the very few Coalition saboteurs on the Hispania sleeper ship somehow making a relatively large remake of the Coalition, among other things.