What are the major differences in building an install profile for Drupal 8?
Apart from specific code changes like converting the .info file to a .info.yml, the biggest conceptual change is that most of what an install profile will contain are configuration files.

Basically, you can take the configuration directory of in installed site, copy it into the config folder of an install profile and then you're already pretty close to building an install profile that will at the end look like the site.

There will still be a few challenges, for example dependencies between configuration and content. Lee Rowlands (@larowlan) is working on a default content module that allows to import content entities from .yml files too https://github.com/larowlan/default_content, but you can't just stuff all of it into an install profile.

Take a custom block for example. You have a block configuration, which depends on  a custom block content, which in turn depends on a custom block type (bundle of the custom block). I'm guessing the way to solve that will be to split an install profile in a set of modules, that depend on each other.