what separates a senior tech writer from a regular tech writer?

[Claire Conant]

Providing a new tier to the job category is a good thing. Not only does it 
give Paul the opportunity to pitch for a raise and additional job 
responsibilities, but it provides others in his group a career path as well.

Flattening the organizational structure is good to an extent when removing 
useless levels of middle management, but keeping some levels in there is 
also important. This is especially true when a technical writing group needs 
someone "senior" who can help lead and drive the technical publications 
processes and provide quality to the written product.

Claire Conant