You need to shape your talk to fit your audience. If you are talking to a religious/Christian group, then the differences in theology as other posters mentioned would be of interest.
If you are talking to a more mixed group, then dicussing the social/ cultural differences(high control religious groups/cult like behavior) will be of more interest. Use the actual Watchtower quotes or book quotes to show what you mean. Let their own words condemn them. As a non-JW, these are some of the things I found most disturbing. .
-once someone is converted to JW, they are discouraged from spending much time with their family if they are not JWs or sympathetic to them, effectively breaking the family bond.
- no birthdays, Christmas, Valentines, Mothers day etc. and why
- the numbers of hours knocking on doors and that it must be reported, also the number of meetings every week
-the dress code for meetings
-treatment of women, very sexist, man head of house, cover head to pray, dealing with abusive marriage and coverups
-coverups of child abuse and continued support of convicted abusers within congregations and that they still go door to door
-elders have enormous power and who they are (plumbers, windowwashers?) and may not have much formal education/training to influence the life alternating nature of their decisions. The incredible control they exert over every aspect (including very personal ones) of the ordinary JW's life.
-even questioning can cause disfellowshipping, or smoking, accepting blood to save their own life, that you don't have to be a bad person or to have done bad things to be DF
-the blood policy and the HLC (what is is and how much medical training they have?) and their intrusiveness into someone's medical care, and that many people have died due to the blood policy
-that JWs are discouraged from socializing with anyone who isn't JW such as work friends or school friends, also not play on school teams
-JWs may not respect confidentiality in their jobs
-discouragement of university education
-that JWs think the world is a terrible place and are looking forward to the end????
-I wouldn't go into much about all the sexual DF stuff because, really, most people don't talk in public about that kind of thing. It is considered inappropriate. I have never seen people so obsessed about sex and talk as much about it as Jehovah's Witnesses.
I'm sure there are tons more, but this is what I recall hitting me when I first learned about the Watchtower.