From the top of my head, maybe the change to Clock app. Before timer, stopwatch, etc. we're in the action bar, but now they're in the three lines section so now you have to tap that each you want to access something different. I'm not 100% sure how I feel about it yet because I haven't used it enough but I think it could be faster having then all on the action bar.
And that's from the top of my head. Honestly I don't remember exactly how everything was pre 10.3. I think there's been changes to maps but I don't know.
Please share so I can see what you're talking about and see how I feel about it. Things like this are awesome so everything is known so things only get better and not worse.
Another thing I want to add is how when you check tasks off in remember they stay in the same place, only that now they're slashed. Before, they would be placed to the bottom on the finished / done section.
Now this isn't something that is different from pre 10.3 because pre 10.3 didn't have quick settings from apps but something I don't like is how you can't access quick settings in landscape or when watching something on crackle.
Another change I don't like. In the Hub, when selecting more than one item, before the delete button was always on the lower right corner, now you have to hit the three dots to get to delete. Don't like that, it takes longer.
Why is it now that when you long press an email in the hub, the full menu flies out from the right including the descriptions, whereas before it was just the icons...