You are still responsible for serving up your own assets, they just need to be served over SSL. I don't think we have a Shopify Widget kit yet, so embedded apps will still look different, and that's fine.
Your app is run from an iFrame, so it's all sandboxed and stuff. No need to worry about Shopifys styles clobbering your UI.
+1 on the widget kit.
For example, CSS for the entire admin interface would be good, rather than making app developers clone it individually. Sample markup for all the admin components would be good, too.
At the very least, a widget to allow apps to use the "Date Range" widget. However, there are a bunch of UI widgets in the Shopify admin that would be nice to be able to work with.
Personally I am glad I am given an iframe and NOT beholden to much of anything from Shopify in that iframe. I have the freedom to do whatever I want in my App. For now it's simple enough to be a Bootstrap clone dweeb but as Sass and JS improve rapidly, it's nice knowing you can adopt anything anytime.
Until some widgets are available, how about publishing a shopify.css with admin styles? So far I've copied https://cdn.shopify.com/s/assets/admin/style-1aed50567ce7c3124c7916f41eed12cf.css, but this is 4mb (!!)