To start things off, here are a few questions to ask.

1. Was this a brand new project, or was this a migration from a previous project? (If a migration, what cms or system were you migrating from?)

1(b) - Additional question on migration - What difficulties did you experience when migrating from another system? Database issues, content issues, training issues, etc.

2. Did you start with a theme and modify it, or did you build a custom template for the project?

3. What plugins did you use and why did you choose those plugins?

4. Where were there struggles in using Wordpress to accomplish the project, and where did Wordpress make it easier?

5. What was type of scope was the project? How long did it take to complete? Were there other issues that had to be dealt with like training of staff to use the site, or modification of workflow?