Coalitions severely reduces the roleplaying aspect of the game. I'm not completely absorbed in the personality of my characters, but I tend to act according to their traits to some extent. While I could wage wars with high frequency with an ambitious brilliant strategist, the succeding diplomatic character might not engage in war at all throughout his reign. I could focus on strengthening relations with my vassals, constructing buildings, setting up strategic marriages, and otherwise remain peaceful.
With the infamy mechanic, this is no longer a rewarding approach. I will have to deliberate each war carefully, and frequently. If I hit 0 infamy, I will be compelled to engage in a war, lest the infamy burn will be ineffective. That means my benevolent diplomat will now have to be just as warmongering as the ambitious strategist, because peaceful reign is no longer rewarding. Conversely, if I exceed the infamy limit, the ambitious strategist will have to remain idle unless I want to fend off half of Europe for pressing a county claim.
Coalitions defeat the purpose of the character driven system of CK2, and every ruler will now be a cautious and robotic machiavellian, instead of actual characters.