Broadly speaking my core complaints with civ5 that could be addressed with patching are in the categories:
(no order of importance)
x) Performance and Optimisation. Things like the individual unit pathfinding with the mouse being too slow to accept a user input (showing a red 'invalid' circle for too long), or turn times taking too long. I don't know if loading savegames can be made to work faster, but I really hope so.
y) No research overflow (yay it's getting fixed
)
z) Lacking MP functionality. In particular the lack of PBEM and pitboss modes. (Hotseat is not something that I personally use, but think should be added as well)
w) Fix any crash-causing bugs, and game-breaking bugs. If there's something like a 10% chance my game will become impossible to continue in the late era, it strongly discourages me from starting a new game. The impression I'm getting is that many of the late-game crashes were related to the problem with the way the game was making savegames, running out of memory and crashing. Crashes are the worst bugs of all, especially for a game that takes a long time to load and then a long time to load a savegame.