I just wrote a really long winded reply to this, but really all you need to know is you are going to miss a lot of applications switching from iOS *IF* you are accustom to using apps which aren't available on WP8; for me this is WP8 missing the ability to VPN in any capacity; for execs in my company, it is missing applications like WebEx that make it a non-starter. Also, if you like first party applications, you'll learn real fast that there are glaringly obvious ommissions: Google Maps, Instagram, SnapChat, YouTube... If you can either live without these, live with the sometimes "hacky" feeling of a third party application, and come to terms with the fact that one day even those hacky third party apps might disappear out of the blue due to threats of litigation, WP8 will be OK, otherwise it should be a non-starter, at least at this stage in its life.
You will also lose access to Apple's support system, which I have no personal experience with, but from everything I've seen and heard, it is pretty great. All of my issues with WP8 and Win8 devices have been handled directly through either vendors or OEMs, and your mileage varies depending on who you get the device from. Lenovo has treated me pretty well, Samsung has not. Verizon has treated me pretty well if I pursue them from the business side (I manage mobility and the "while I have you on the phone, could you help me with..." trick has been a godsend), but as a consumer customer, they have not. I guess the point here is that there is no Microsoft support I have engaged, because they are just responsible for the software, and as such support channels are sort of all over the place. Give this a year or two... now that Nokia devices and services have been obtained by Microsoft, I expect the next evolution of what is now my Lumia device to be that much closer to the Apple experience; I also suspect more first party devices other than the Surface Pro (crippled by battery life) and the Surface RT (crippled by the inability to execute regular x86 applications), which will push the companion device experience in a similar direction.
If you're OK with the growing pains, jump on board my friend, because even though it's been a bumpy ride, I have not regretted my full immersion into the Microsoft ecosystem. Just know you are jumping into an ecosystem significantly younger than the well developed and refined Apple ecosystem you are considering leaving; you're more or less about to break off your long term relationship with a conservative and intelligent girl who is probably starting to seem a little boring in spite of her reliability and support. You're leaving her for a girl who is younger and more exciting; who likes to be spontaneous, flashy, daring. While she'll give you things you'll find yourself infatuated in that you were missing before, she'll also be immature at times, sometimes unreliable, or ignorant to some of the harsher realities of the world.
At the end of the day, the WP8 does for me what a phone needs to do, it does it out of the box, and it does it well. It plays nicely with its PC and tablet cousins, and like watching a child grow up, sometimes you don't realize how quickly it is maturing until you reflect on the past and where it all began.
I'd recommend spending some time with it before going all in. I don't know many iPhone to WP8 users, but I know plenty of iPhone to Android users that ultimately hated their decision. It's not about which is better, it's about which experience you prefer.
tl;dr - WP8 is young and immature compared to Apple and is missing some of the niceties like a single, first party support for software and hardware all in the same place, or the bevvy of applications available on the app store. It's not all bad news though, as WP8 in and of itself is alive with personality, speed, and functionality, not to mention great features out of the box without scraping the app store for necessities. WP8 plays nice with other Win8 devices, and if what the MS ecosystem has to offer you is what you want, dive in.
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