I was an Iphone girl, and have missed certain things. I don't think the S3 is QUITE as perfect with screen sensitivity and also some things just aren't intuitive. I've had to download a lot of apps to do things the stock applications just don't do. In saying that, the reason I switched was because I was sick of Apple locking everything down so tight I couldn't even have my notifications the way I wanted. I love the fact I can have everything on my phone the way I chose! I thought I may pick up the Iphone 5 when it comes out, but now i don't think so, I may be an android convert! I picked up my husbands iphone today and the screen looked tiny!
The camera app annoys me intensely. It seems that you cannot get to the settings menu (eg to change the resolution) without removing an old shortcut and putting on a new one, then when I click back onto the camera, it has reset to default. GRR. I don't need an app to do fancy things, but this seems very user unfriendly. I also hated the samsung keyboard, I quite liked swiftkey but am now in love with swype (now called touch pal strangely!) It's so intuitive and I think much better than ios with punctuation and numbers all on one screen.
I find the phone a bit plasticky and feel more comfortable with it in a case! Maybe this is me being used to the iphone weight.
One thing I don't like about android is I would rather pay 69p or ?1 for a game and have it without ads, but that's never an option due to the piracy. Bit annoying, I had all my iphone games ad free and I'm always accidentally clicking on the ad banners.