I like my Icon about 1000x better since 8.1. The only issue I had was an occasional delay in the lock screen coming up, which the update a few weeks later fixed.
At the top of the list, the notification center (while not yet perfect) eased my biggest frustration with WP8. This was for me truly the difference between hating my phone and loving it, because I didn't have to leave what I was doing to see what just happened.
It could still be improved. For example, I wish it allowed me to clear individual notifications, such as clearing emails, right from the notification center. If I click on an individual email and swipe, it wants to clear all the notifications. I would like to be able to delete an individual message (or at least clear the notification and mark the email read) with that swipe. Also, if I click on an individual email, see it's unimportant and delete it, why would it take me back to the notification center if there are other unread emails.
That and I do get some use from Cortana, although she is clearly a work in progress. The geofencing thing, for example, takes a while to kick in. I have a short commute from home to work, and it takes me past the dry cleaners, which is about halfway. I asked Cortana to remind me to pick up my dry cleaning next time I was leaving the office. The reminder would faithfully show up as I was arriving home sans dry cleaning. And I wish there was a way to snooze the reminders. I wanted to remind myself to look up a website when I got to the office, and she did remind me, but I couldn't do anything with the phone unless I told her either next time or complete. I just wanted to be reminded in fifteen minutes or so, after I retrieved the number my co-worker was looking for as I arrived.
Small changes all the same, and Microsoft will doubtless be speeding up improvements from this point forward. When I first got my Icon and friends asked me how I liked it, I told them about the things that I liked but couldn't recommend it to anyone with an iPhone or Android because of so many compromises. Now, friends are starting to take a look and I feel much more confident about suggesting they check the Icon out.