For iPhone developers (ignoring iPad for now) with an emphasis on getting the fastest app store approval. Everything that was a suggestion becomes an imperative where it makes sense. If you're thinking of violating these rules, you can check for yourself what the caveats are. Generally speaking, deviating from these imperatives will more likely cause your app to be hung up in approval.
You can read this entire document in about 20 minutes. This is faster than reading and understanding the entire Human Interface Guidelines.
- iPhone apps home / start screen shall be portrait mode (11)
- Don't use standard gestures to do non-standard things (11, 60)