I wouldn't put too much worry into it.
The vast majority of the changes are structural, in other words, the naked eye can't see them. I'm going to assume you haven't seen/read FIAT 500: The Autobiography. Checking that book out (maybe at your LS) will put any worries you may have too rest.
Exterior: Passenger side windshield wiper & headlights are two things the naked eye can see. The EU wiper set-up is more pleasing aesthetically, while Giolito added a cool accent to the NA headlights. Then, there is the obvious change/s to the front & rear fascia (to accommodate NA license plates). Those are design changes the naked eye should be able to see, without the use of a tape measure. In NA, we have the undercarriage spare set-up. It's in the boot of the EU version. In NA, the tail pipe chrome tip is standard since 2012 ~ not so in the EU.
Interior: The armrest was added & the seats have more cushion to accommodate the longer commuting times in NA. The EU power window (an expensive upgrade in Europe) buttons are square and devoid of the chrome accents. No factory-heated seats option in EU version, so the buttons are added as opposed to be an integrated part of the interior. The button set-up on the steering wheel is different, despite the fact it looks very similar. The EU 500 is noisier and bumpier. The console is slightly different ~ there is a small compartment on the NA has the net pocket. The EU version is devoid of a glove box door. In NA, the leather-wrapped steering is standard since 2012 ~ not so in the EU.
Under the bonnet:
There are a host of differences ~ the EU has quite a few. The main difference is the NA Multi-air. It will become an option everywhere in the 2015 global 500.
So, as you can see, there really isn't anything to worry about. Giolito was the head of the team which developed the NA 500. Is has been noted, in print and in recorded video, he took advantage of the need to make regulatory changes for NA. He made improvements to the car. In fact, NAs are (basically) driving the 2015 global 500 now.
I just noticed noticed you checked out mocha's link. I'm glad you're relieved.