I'm another long time iOS user who just switched to the One. I would probably have upgraded to the iPhone 5 but I'm not really a fan of the elongated form factor and I wanted to try something different.
There's a big learning curve jumping from iOS to Android. A lot of the things I have been frustrated with on the One have mostly been from being firmly entrenched in the "Apple Way" of doing things. What I will say, however, is that a lot of the time the "Apple Way" is a really good way.
The default music player, as well as Google Play, is infuriating to me. The one in iOS is just *perfect.* I have a lot of music on my phone, so I want to see a simple list of artists, then under that a simple list of the albums I have, and then under the albums a simple list of the tracks. The Sense Music Player does that (kind of - you can't drill down into individual albums because they're all automatically expanded) but what really gets me is that it sorts bands that start with a "The" in the "T" section. I can't believe that, in the Year of Our Lord 2013, that someone would still code a music app that way. The solution for me was to pay for a license for Poweramp to (somewhat) replicate the iOS music app.
Another gripe I have is Blinkfeed. Why can't it just be a widget that you can disable? I potentially could see myself using it if there were more content sources, but I'm not really interested in the ones they have. Ideally I'd just disable all of the facebook/flickr/news stuff and have it show my calendar, but it complains at you with a big blue box if you don't pick something else for it to show besides your calendar.
Same thing with the Gallery app. I'd just like to see the pictures I've taken with the camera phone. I don't want to see my friends Facebook pictures in there.
The other thing that kills me is the flush power and volume buttons. It's annoying to change the volume if you're walking and the phone is in your pocket.
I've had the phone for 16 days so far and just recently a spec of dust got under the camera lens, so there's a transparent orb that shows up on every picture I take. I had a rather unpleasant experience at the AT&T store today dealing with it, although they are sending me a refurb HTC One as a replacement. (Not so happy to have bought a new phone 16 days ago and now end up with a refurb, but a working refurb to me is better than a broken new phone.) Truth be told, had they let me, I'd have just exchanged the phone out for an iPhone 5, but it seems like I'm stuck with this phone for the next 2 years so I may as well make the best of it.
I'm hoping for a sense-less HTC provided ROM soon. I think that will go a way towards making me a little happier with the phone.