I don't like what is perceived to be a very authoritative and assertive management style and a lot of their marketing tactics. A good example of this is their claim that they are always "No.1 for Customer Service", and then basing this on "Flights on Time" and "Lost Bags" - as if these are the only two things that are important when considering customer service. Their actual face to face and telephone customer service is, in my experience, pretty hopeless.
Despite all this and also the lack of professionalism demonstrated by Ryanair cabin crew I have witnessed on various occasions (laughing during safety briefings, shouting at passengers, etc) I will continue to fly with them because even with all the extra charges they tack on to their basic fares, they still often end up much cheaper than any of their competitors (even Easyjet); their "out of the way" airports are often actually very convenient for where I need to go; and importantly, nobody can deny they have an excellent safety record.