Sorry, long-winded post here, but I am just trying to be thorough so you have an idea what switching is like and what options you have.
I have an iPhone and am largely in the Apple ecosystem, but I purchased a Nexus 7 to compare to my iPad. The tablet transition is probably less of an "ecosystem shock" for a number of reasons; however, I still have some insight into the process.
The experience of switching to Android will vary considerably depending on how much you rely on Apple specific services. The problem with iCloud is that it really just isn't ideal on any device outside of one made by Apple. If you want to gain any measure of platform independence, I would recommend switching away from iCloud (or at the very least, mirroring your iCloud stuff to a different service). Google's services are the most obvious choice, but there are others as well. I actually kept my data in iCloud and continue to use it so that everything syncs to my Mac from my iPhone, and I also keep a copy of my contacts, calendars, etc on Google for my N7. This increases the amount of maintenance you need to do to keep everything in order across two services, although there are some automated utilities that can handle it too. Here are a few tips (I'll focus on Google since I believe that is what most Android users choose, but these steps could equally be applied elsewhere):
-Contacts: They are very easy to move over, just export them as VCards from your OS X Address Book and upload them to Gmail's contact list. You can also set up CardDAV syncing with Google on any of your iOS devices to keep everything syncing between iPads and whatnot and your Google account (and therefore your Android device).
-Calendar: I don't believe there is a way to get iCloud calendar syncing on Android, but I could be wrong. Google Calendar works well on Android, and there is CalDAV syncing available for iOS devices. The one casualty is that shared calendars don't work across iCloud and GCal. There may also be some quirks syncing Google calendars to an iOS device.
-Email: I personally use Gmail, which automatically gets configured on the Android device when you sign into your Google Account, but there may be a way, albeit a complicated way, to get iCloud Mail on there if you use it.
-Music: I would definitely second Hallux's recommendation on installing Google Music Manager, pointing it at your iTunes library, and letting it keep everything in sync. Google Music works well in my opinion, and it is very nice no longer having to manually sync media over or have all that music taking up space locally on the device.
-Photos: This is going to be the tough one. I know of no way to get Photo Stream or shared photo stream albums on Android. However, there are some workarounds off the top of my head involving automatic backup on your Android phone to either G+ or Dropbox (you can specify on Android to automatically upload photos you take to a third party app like Dropbox if the app supports it, without you having to open the app every time to get the upload started like on iOS), then grabbing those photos on your Mac into iPhoto and photo stream from there.
-Apps: Android has all the apps I regularly use available on the Play Store, even if I feel that there are several apps that either outright work better on iOS or just feel better on iOS. I don't think it is a huge problem, but you'll lose those apps you have invested in on your iPhone. If you are keeping an iPad, then it won't be a total loss. Check out the Google Play Store on the web and make sure all the apps you need are there.
-Browser Sync: As there is no Safari available for Android, your best bet here is to download Chrome on your Mac and your iOS devices, move your bookmarks to Chrome on OS X, sign in with your Google account and turn on Chrome sync on each of your devices, and let that handle your browser syncing needs.
-Various Apple specific services: You may miss these most of all. iMessage, Find my Friends, AirPlay, Game Center, the simplicity of just using iCloud rather than having to also keep Google stuff current, AirDrop in iOS 7, Safari tab and bookmark sync if you use Safari on your Mac and iOS devices, iWork document sync, etc. I think there may be a few Android apps that claim to allow your device to use AirPlay, or you could get a Chromecast (doesn't totally replace AirPlay, though), but I haven't investigated any of that in any great detail. Just be aware that you'll likely not find complete replacements for these things. If you can convince friends and family to use Google Hangouts or FB Messenger or whatever other chat service, then the loss of iMessage may not be as bad as it would otherwise. Other than that, it just takes an adjustment period to get used to the lack of those Apple bits. You'll have an iPad to continue to use these services on, but do remember to disassociate your phone number from iMessage on your iPhone before trading it for the S4.
