What is different is that it has been updated to support the 802.11ac Wi-Fi standard. According to a press release the "Linksys WRT1900AC Dual Band Wi-Fi Router is inspired by the original WRT54G iconic blue/black stackable form factor” however it has a dual-core 1.2GHz processor, four removable antennas, instead of three, and eSata and USB 2.0/3.0 connectivity ports.
Four gigabit LAN ports are included. Maximum throughput will be "up to 1.3Gbps on the 5GHz band and up to 600Mpbs on the 2.4GHz band."
The WRT1900AC is also the first Linksys router to include a Network Map feature designed to provide a simpler way of managing settings of each device connected to the network. The $300 might seem a bit steep but Linksys said that was because it was the most powerful router in its class on the market.