To begin with, some basic tasks are faster. Keyboard shortcuts help you navigate around without as much mouse clicking, and a dedicated emoji button can add some smilies or hand claps to your latest tweet. You'll also find an upgraded trend section for times when you want to track the planet's pulse, and search promises to be more sophisticated.
This doesn't appear to include some of the big updates Twitter mentioned recently, such as status updates. And no, there's no edit button. Still, it's notable that Twitter is giving its web client an overhaul after focusing so much of its attention on mobile users.
Just when you'll see it is another story. At the moment, it's limited to a portion of the audience through an opt-in button. You'll know if you have access, then. It could be a while before the makeover is broadly available.