The down side is that players on those servers will be experiencing several days of downtime; players who had characters on those worlds before today and have an active subscription will be compensated for lost play time, which is expected to be anywhere between three to five days. Players on those servers will also be given the option to transfer off for free. You can check out all of the the details on the official site, but the practical upshot will hopefully be a more pleasant play experience for European players.
Final Fantasy XIV is not a game with action combat, but you can still be pretty thoroughly screwed whilst trying to play the game if your connection is bad. This has been a major problem for European players, since the game’s server clusters are located in North America and Japan. So European players will likely be happy to know that the game is moving one of its data centers to Europe, allowing players on the Chaos server cluster to have a more local and less laggy play experience.
Source: Official Site