Don't let the fact that there are possibly dozens of similar/same apps as one you might think of creating. There will always be a market for something that's better or different because there are people with different tastes.

I have tried a bunch of apps that sound like they'll be amazing but then turn out to just fall short of the mark.

Fortunately for Developers, Apple has left a lot of room because all (I do mean ALL) of their default apps suck complete trash. That is, as long as they don't start denying apps that compete with theirs again.

Now hopefully iPhone OS 3 will give us a way to hide items from the spring board. Then I can get rid of half of the useless "apps" Apple builds in, even if only to prevent seeing them listed. Then I hope we see some quality replacement applications, such as a music player - the iPod app is so so so bad it's not even mildly humorous.