What features any good application should care about 'By-Design' [closed]

Question

I know that the default answer is "it depends", but I want to know the feature list you are working from in developing a good application.

I'm particularly interested in features that need to be cared about at design time because adding them late will cost us a lot.

Please list one feature per answer so we can vote for the most important features.

Any extra info, like your own war stories, will be appreciated.

I ask this question to enhance my application from day zero, and I think I'm not the only one with this target, so please write whatever feature YOU think should be exist in a good application.

Answer 1

Logging and error handling

Answer 2

Security. Pretty amusing, if unsurprising, that this is the fourteenth answer to go up.