One particular thing that happens a lot is that these people forget the structure of Vodou. In Haitian Vodou you are to go to Legba, give him offerings and ask him to open the door for you before you do anything. Without Legba's permission, you can't go far in the religion. Instead these people go straight to (insert spirit here) and wonder why they've gotten no results. Considering the spirits that many try to get in contact with, they should be glad they got no answer.
Another thing is that many of these folks approach Vodou are not comfortable with the idea that the Lwa are real beings that can affect our life in both positive and negative ways. They don't like to accept the fact that if they approach the spirits they might decide to talk back (or teach them a lesson). Instead they label them as "Archetypes", throw some words they learned out of a book into the mix, have a bad experience (because of the disrespect) and then go around saying that the Lwa are demons or something similar. Now this is not to say that none of the spirits in Vodou are wrathful, they are but if approached with commonsense and respect it would cause less problems.
Then there are people who like to use Vodou as a "money maker" idea. They craft bottles "for the Lwa" and make dolls (most of the time stuffed only with cotton...where's the magick in that??) then sell them to people who think they are getting real spiritual items. Its in bad taste for sure, but it does happen and they do tend to get away with it.
If the individuals who are curious about Haitian Vodou would just study hard and approach the Lwa sincerely they would have little to no problems with the spirits. But they don't, instead many decide that the Lwa don't expect anything in return for what they do for others and that the Lwa "can't harm them". This is just one of those things that angers me and while I know it won't change anytime soon I can at least give one last statement:
Vodou is a very beautiful thing, but it is as real as your heartbeat. If you have a sincere respect for the spirits and the tradition Vodou is a life changing thing.