Unlike Trident, the new browser engine is designed to be updated, which allows Microsoft to keep its new browser current in a way that was impossible with IE. Maximum possible score is 555. Project Spartan will provide a level of plug-in support similar to Windows 8's Metro IE browser.
Building a search-based agent into a browser makes a lot of sense, especially when it's able to use the context of your searches and browser history to make inferences about what information you need - initially giving you weather and stock information.
Microsoft is putting Project Spartan front and center in Windows 10, with the new browser pinned to both the Start menu and the Windows task bar.
With only a week or so of development time separating this Windows 10 release from build 10041, Project Spartan is not merely the big news but the only news in this release.