Things to be considered for a large scale SAAS products

Question

Hi How ar you all? I would like to ask a general questions to the expert who have done large scale Software application web & mobile applications works as Software as a service.

Which specific generic things a large scale SAAS product should have like for error logging it should have complete error logging control like elmah , what else for web and mobile which constraints someone should take care off, how to load test the mobile and web app .