I'm noticing this to and I'm on my 2nd week with my S4, switching from iPhone 4S.
The apps I use are mostly avaiable, but they seem to be a version behind the iOS counterpart.
For example:
Zite - looks like the version before the UI makeover on iOS which is way better
Snapseed - doesn't support the S4 camera and compresses my files to 3MP!
Aviary - supports only up to 12MP, but saves the photos metadata. iOS photo apps LOVE to strip metadata from files (which caused me all kinds of issues in my iPhoto organization). The Aviary apps saves all original metadata but the iOS does not.
Facebook - no drop down menu to view only Photos, Music, Posts, etc.
Instagram - doesn't auto rotate my photos and must rotate myself (if I don't remember to take the photo with the volume rocker facing down). Also, no one tap auto enhance for the Android app like it is on iOS.
Apps I am missing from iOS: VSCOCam, Oggle, Hipstamatic, Day One
I've replaced my go to photo processing app with Pixlr has the filters are great and supports the full resolution of the S4. Apparently VSOCam is coming to Android, but only god knows when.
I will say that Android apps allow you do to more. For example:
Instagram - has a high quality mode in the Advanced Options. And you can use your default camera instead of Instgrams.
Dropbox - automatic uploads work in background without me having to open the app first. Also, I can create albums right on my phone - an option not available on iOS.
There are a lot of pros and cons I'm finding with my switch to iOS. I'll be writing a review soon so stuff like this will be in there. I believe it things I haven't seen written about before and authors kind of gloss over.