Application Code:

Maintainability
The code should be well commented and structured so that maintenance and troubleshooting are efficient and can be done by any competent programmer of the relevant language.

Scalability
The code should be multiple-tier architecture (like MVC - Model/View/Controler). Steps should be taken to ensure substantial growth in the use of the application wouldn't adversely impact performance, maintainability, security, etc.

Code Auditability
Developers of critical systems must ensure their applications are not only auditable to standards and best practices, but should have a plan of auditing the application to ensure continued compliance. For example, review user accounts every three months, verify authentication controls are working, run checks on the database to make sure nothing is in the database that shouldn't be in the database, etc. This includes having good documentation of the application inside and outside of the code.

Performance
Easily the most noticeable and most easily detected problem by non-technical users. Good multiple-tier architecture, adherence to standard code conventions and good database design (normalization) will typically take care of performance.