APPS AND GAMES:
You are probably going to miss a few games if you are a heavy gamer else you won't notice.
Some niche apps may be missing but that's a rare case unless you want apps to throw out your garbage, winetasting, fart sounds etc etc.
Some official DEVS have found a hobby of being a **** and not pushing out updates (WhatsApp, I'm talking about you here).
Some official apps may be lower in functionality compared to iOS. (Facebook and WhatsApp only)
In the app department, games may be missed, but you will rarely have any other problem.
STABLITY:
One of the most stable and frequently updated OSes around. NO RANDOM REBOOTS (if you keep your nose clean), no lags, no crashes (barring on the cheapest Lumia 520).
SECURITY:
In terms of consumer viewpoint, it is the most secure. No known vulnerablity till date. In terms of corporate, some things may be missing. Some companies don't allow WP as a BYOD.
CAMERA:
Get a nokia and forget a digital camera. Get the 1020, forget a DSLR.
USER EXPERIENCE:
Fluid, lag free, intuitive, minimalist design.
(You may complain about the updates Music app).
Cortana!!! Better than Siri at MOST of the THINGS THAT MATTER to an AVERAGE guy. (No WolframAlpha here.)
Live Tiles! (A novel way to present info)
I don't know about on-contract phones cuz I live in India. So I will tell you prices of unlocked phones:
Best budget phone (for a first time smartphone owner ONLY): Lumia 525/Lumia 630 (150$)
Best mid-range phone (avergae Joe): Lumia 920/925
Best high end phone: Lumia 930/Icon
Best niche devices: Lumia 1020/1520
Follow another compatriot's conversion to WP from iOS and his musings. Maybe you may relate to him
http://forums.windowscentral.com/windows-phone-8/263879-considering-trying-out-windows-phone.html