The 2 biggest changes (IMHO) are the hex grid and 1 Unit per Tile (aka 1UPT). Makes combat a very different (and more tactical) prospect and I don't think I could go back to stacks of doom after playing Civ5. (Another big change is that you can no longer trade techs (or capture them if you recall the older iterations). They have something called research agreements which allow you speed up your research on a deal-by-deal basis, but no more "once one computer opponent gets it, all have it" nonsense.) The downside combat-wise is that the computer players handles war poorly because the AI isn't quite up to it. I think the upsides to the changes in combat outweigh the downsides. I recommend it as a worthy upgrade to Civ4. EDIT: Especially at 66% or 75% off.