Assuming you're not talking about lens effects (which definitely has a huge effect), you might be reacting to the use of tone mapping in iPhone photos, which te

Assuming you're not talking about lens effects (which definitely has a huge effect), you might be reacting to the use of tone mapping in iPhone photos, which tends to make things look "flat". There's a slider called "Brilliance" in the iPhone photos app that you can play around with to get a sense of how this type of processing tends to flatten images. I'm not sure how or whether it's possible to disable this in the default camera app.