One starts with the words "The worst thing about".... then names the topic, and thereafter explains why something is so unpleasant/unacceptable etc. about it.
Heres a few examples
The worst thing about the Police is that the only ones good enough at handling weapons, are the criminals themselves.
The worst thing about defense Lawyers, is that the amount you have to pay them, will probably end up being more expensive, than the amount you're being sued for.
The worst thing about School, is that you only realise once you've left, that the things you learnt are of no importance in the real world.
Now your turn...
Remember your explanation will most likely be an opinion rather than fact.