I can build a site using SharePoint. The concepts on which I can work on are :
Create sites from scratch.
SharePoint Designer Workflows.
Page Layouts.
Master Pages.
Event Receivers.
Custom Timer Jobs.
Custom List Views
Search
Metadata
Powershell
REST API
Client Object Model, Server Object Model, JavaScript Object Model

I can work on SharePoint 2007, SharePoint 2010, SharePoint 2013 in both the environments, on-premise as well as SharePoint online.

OR

I can also perform best practices on Existing Sites:
1) Remove unwanted files
2) Remove missing assembly entries from the database entry
3) Removes missing file references from the database entry
4) Remove missing features from the database entry
5) Check for memory leakage in code
6) Configure error logging in code in case not implemented