"Customer Focus" .. Yes the answer is Customer Focus .. Everything else come after that.. You see as a tester, You have the responsibility to make sure, Once the product goes to customer, He is able to get his business done.
Let me give one example.. You are testing a VOIP software for a call center. it generic software which can be customized by Call center company based on their usage.
You have to think from Customer point of view..
1. Direct customers: Like Airtel CIO, IT Heads of Call center companies. He will talk about Reliability, Scalability, Support etc..
2. Indirect customers: Call center executives, Who will operate this software.. You must know, Their IT skills will not be as equal as IT Head, Or Yourself.For them its the features .. How quickly he can get the caller information.. ?
3. Customer's customer: Callers.. General public .. What they would need is quick resolution of problems, Voice Clarity etc ..
You as a tester should wear different customer Hat and question to software like that .. Are you reliable .. Can I measure your reliability?
You might be knowing that, 20% of Features of a product gets used 80% of the time.. So Identify those functions.. and test them thoroughly..
You should participate in Customer Beta Feedback, Forums etc.. To understand Customer's issue and try to have testing for them ..