Fr. Longenecker has a great list up on Standing on My Head. It covers the things he tells Protestants who question him about the Roman Catholic Church, more specifically the ones who are considering Catholicism but have a few hang-ups.
As the sister of a mega-church-attending, fallen-away Catholic I see the list as complaints I've heard from my brother about a faith he never cared about before so he never took the time to understand.
A couple of my favorite points that Father makes:
5.I know it seems like you're "not getting fed." Once you're received into the church you'll be fed alright: Fed the Body of Christ. Be patient.
8. You wonder where the 'fellowship' is. That's because a typical Catholic parish is huge and socially varied. Protestant churches tend to develop congregations from the same socio economic background. No wonder they have 'good fellowship'. In the Catholic Church you will find your 'fellowship' in one of a huge number of sub groups. Once you've swum the Tiber, join the Women's Club or the Knights of Columbus or the Pro Life Group or the Carmelite Prayer Group or the Third Order Franciscans.
One thing I've noticed about people who leave the Catholic Church is that they often leave for a denomination that is much more "feel good" and "easy." It's so much easier to believe that once I've accepted Jesus as my Savior I'm good to go. It makes me feel loved and comfortable. Being reminded of my sinfulness and need to constantly repent (confession), and my God-given command to take up my cross and do good works - well that can just make life a lot harder. Plus, the praise and worship rock band is so much better than an old hymn and a piano. And there's a coffee shop and bookstore in the lobby at the mega church, unlike the bulletins and holy water font in most Catholic churches.
You may think I'm being harsh, but these are real statements I've heard from real people about why they have left the Church.
I go to church to receive the Eucharist - I need it. But more than that, I go because God commanded me to go, and I choose to obey, and because He is worthy of being praised, worshipped and adored. With that kind of attitude, the coffee shop, fellowship, and good music that I enjoy aren't very important.