We are looking to rework our notifications system, hopefully next year. There are a number of issues with regards to lack of customization that we would like to solve. The problem is right now, to make this change so only show X characters of a post would require that we made that change for everyone. This is problematic as some people like the whole contents in their email, it lets them read it without going into the community and makes for an easier time on mobile. We even offer the option to respond to the email to comment. This is all done intentionally so that people can work from their email. Even if we had a cut off, some sites write longer posts than others, so it makes it hard to decide if we should cut them off, if so where.
With the notification system rework we would be looking at expanding those options so that individual sites change stuff like branding their notifications or setting text cutoffs. This way we can pass the decision making onto the client and what works for their site and organization.
I am afraid until then the notification will contain the complete text as to not restrict access to the information for those who want it.
Hope that information is helpful!