Hi Muhammad,
I am not familiar with other rm tools, but some things that may be relevant -

• The ease of creating links for traceability/dependancy - links among requirements, links between requirements and external URLs, and if relevant - links to RTC work items.
• Need to exporting requirements into doc/pdf/html.
  
• Need of commenting on a requirement, including directing the comment to a person that will get email notification.
• Need to store other documents (e.g pictures, ppt) as part of the requirements.
  
• Is there a need to develop programs or scripts that access/create/update  requirements from outside - I don't know how it is in other tools. Doors NG have good interface for that.

Also - If you want to migrate the customer to a new tool, need to address the issue of importing requirements into Doors NG - it is possible to import from doc and csv, but if the requirements are already in a different tool, it may be hard to import, as well as adapt the processes of working with requirement management to a new tool.