The Elgato Thunderbolt™ 2 Dock and the original Elgato Thunderbolt™ Dock are similar, but there are some important differences:
Types of Thunderbolt Ports
Elgato Thunderbolt™ 2 Dock has two Thunderbolt™ 2 ports, while the original Elgato Thunderbolt™ Dock has two Thunderbolt™ 1 ports.
For both Docks, you will have one free Thunderbolt™ port, since the other of the two ports will be connected to your computer.
4K Video
Elgato Thunderbolt™ 2 Dock can deal with 4K video.
The original Elgato Thunderbolt™ Dock cannot deal with 4K video in most circumstances.
See this article for more about 4K video and Elgato Thunderbolt™ 2 Dock:
Can Elgato Thunderbolt™ 2 Dock use 4K displays?
Amplified Audio
Elgato Thunderbolt™ 2 Dock has amplified audio out, while the original Elgato Thunderbolt™ Dock has a quieter audio out port.
USB Charging
Elgato Thunderbolt™ 2 Dock can charge things via USB, even if your computer is not connected to it. The original Elgato Thunderbolt™ Dock can only charge items if your computer is connected to it.
See this article for more details:
Can I charge USB devices using Elgato Thunderbolt™ 2 Dock?