Pluspunten
Your working experience varies by department or team. The culture and attitude is definitely "can do," "get it done," which is great! Aon has great career/employee resources in place, you just have to look for them. It's easy to find mentors and interesting projects in which to participate. Promotions don't come easy. Having great working relationships with management and demonstrating the ability to lead and getting things done will go a long way. Overall, Aon is relatively stable, very structured and global. You will work with very smart people. Aon is definitely a resume builder if you're fresh out of undergrad.
Minpunten
As with most companies, office politics can play a huge role in the day-to-day work. Depending on your department/team, it can be difficult to make an impact because 1) someone already tried/has done it 2) understanding the global implications, while critical, is time consuming/expensive 3) it takes just one of the many CEOs/COOs/CIOs etc. to derail your project 4) the existing processes/controls can hinder getting anything done.