Which is the most important? And to who?
Performance is important. Obviously an increase in speed will increase the amount of data that can be processed in a certain amount of time.* Which is a good thing, right? But is it the most important? I wouldn’t think so. In most applications the amount of time the user spends inputting data is far greater than the time saved by most optimisations, so it goes mostly unnoticed.
Ease of programming might sound unimportant, but it also includes ease of maintenance, which is important if users want to have new features quickly, or at all. Additionally, the easier the program is to create, the faster it can be created. Which is good, right?
Ease of use is again obviously important. Usuability, accessability, etc. is a major selling-point for any software package, simple because a lot of other software is very difficult to use. It can be difficult to create easy to use software. Is it the most important? Probably. An easy to use program with a very slowly increasing feature-set would presumably be considered better than a feature-heavy, constantly-updated program that is difficult or even impossible to use.
Ease of Use, Ease of Programming, Performance. Pick Two.
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* This is all that any program really is, a data processor.
