If you are in the software business or thinking of getting involced in a startup I suggest that you sit down and read this article. There are several excellent answers to the question "Why is dropbox so successful?" including one from the co-founder of a competitor. There are so many great points made, but this one resonated particularly strongly.
If you're starting a new company, the best thing you can do is keep your feature set small and focused. Do one thing as best as you possibly can. Your users will beg and beg for more functionality. They will tell you their problems and ask you to fix it. My philosophy is that they're right if their feature request is right only if it works for 80% of your customers. Until you have a lot of resources, stay focused on your core competency.