We love using Solve and the way my team has adopted it eagerly is testament to its intuitive approach.

We install renewable energy systems, and for most of our project work the section headers work well, allowing sensible grouping of documents.  However for larger projects, the number of documents within the project can grow rapidly.  For these situations, it would be really useful to have a Sub-Section header which can be linked to a section header.
So we could have:

Project Documents (Section Header)
  Design Calculations (Sub-Section Header)
  Bills of Materials (Sub-Section Header)
  Variation Orders (Sub-Section Header)

You get the idea.

Our use of Google Docs is working really well with Solve, especially GDocs handles version control really well, allowing Solve to stay as clean as possible.  Having sub-sections would be the icing on the cake.