Is our site configured correctly? How can our site be configured like Stack Overflow requiring 1500 rep? Is there any chance that tags created by users with insufficient rep, be removed for now?
The one you're looking at is Stack Overflow's privilege, which has
graduated a site-specific design, and thus they have "designed" rep threshold. Moreover, the threshold for creating new tags on SO is customized to 1500 rep while normal sites only needs 300.
This is by-design, as mentioned on this site's graduation post:
The site will still receive a full design from one of our designers, which will be packaged with an increase in the amount of reputation needed to access each privilege.
For more information: FAQ - Reputation requirements compared.