One flaw I don't like in CK2's interface is that there is no 'Friends' and 'Rivals' category in character information panel. That thing was very helpful, and you could quickly check your rival if he tried to assassinate you, or you could get your friends to help you out in many tasks. In other words, it was good to keep track of friends and rivals.
Now there is no 'friend' or 'rival' category. They simply gain or lose relations with you. And you can never figure out which poor-relations character out of hundreds of such characters tried to assassinate you. Neither you can take note of your special friends, because they just get temporary relations for some time, which is not friendship. Lazy work on Paradox's part, methinks.
I still love the CK1 portraits. And especially the dark-age style music and sounds. Don't know why they removed them.