SharePoint framework would give you the experience which you have developed solutions client side such as Angular, Ember, JQuery, Knockout or etc. TypeScript will give you thinner layer between the SharePoint server and client side framework but it also not a mandatory fact.
Since its loosely coupled with the SharePoint server, it will be easy us to make upgrades easy and do the maintenance on presentation layer. Also it allows us to integrate power of the modern client side frameworks with greater user experience.
All about client side scripts
All resides on the clients browser and it will give the same behavior while your felt with client side web development. You may be must to scripts enabled on the browsers to consume this services. It will include the strengths of add-ins and frameworks.
SharePoint framework would take your current browsers authentication to authenticate against the SharePoint server. Other than in web parts and in apps model, you wont equipped with authentication impersonation.
With the decoupled characteristic of the framework, you will be allowed to host your content where ever you want. It may be cloud, on-premise server, private host or any form hosting since you have access to URi’s of the SharePoint.
As Microsoft announce itself, this would be a extensible module for the SharePoint user experience. You will be totally free to include modern features such as responsive, nice animations, smart design architectures and so on on your preference. Rather than we develop within the boundaries of the template we get. This will be tend towards opening doors of open source.
Selecting to do your customization is more or less selecting best comparing competitive advantages. If you selected to develop on this some how, SharePoint would play the role of API.