There are three things that stop me from switching to Android right now and they are iMessage, iTunes, and Tweetlogix. Everything else I can pretty much do without.
To be quite honest, iTunes isn't an issue at all. I still use it to purchase music and have all my music synced over iTunes match (on my iPhone 5, iMac and iPad mini). I bought a couple songs yesterday on my iPhone - a few minutes later, viola - they were right there on my Nexus 4.
Just download the Play Music manager to your computer, make sure you have iTunes set to auto download anything you buy from another device, and set the iTunes folder as the one to pull music from. Then when you buy a song:
-Song bought on iPhone > downloads automatically to iMac > goes into iTunes music folder (I have it set to auto manage) > Music Manager sees a new song in the iTunes folder > uploads it to the Play Music cloud > ready to play on nexus 4.
Its pretty sweet.
As for iMessage, that is my biggest issue atm. I have radiant barrier in my house and despite the excellent cell reception outside, I get pretty spotty weak reception inside. I'm researching some boosters to fix the problem, but texting over wifi through iMessage was a big deal for me. You can pretty much mimic any of the other iMessage features with other apps like Mighty Text (allows you to text from any computer).
Having never used Tweetlogix, I can't say for sure - but Falcon Pro is a great twitter client for Android. The widget can be used as a live twitter feed itself. I also like the layout and design.
Anyways - take it from a longtime Apple/iOS user (and someone who still prefers iOS to Android) - the switch isn't all that difficult, and I find it quite rewarding using both platforms side-by-side. Best of both worlds!