1. To start with, they sneaked in the request to access all your email contacts and constantly spam them in your name which can be embarrassing at times to say the least.
2. Their mobile apps do the same with your phone contacts, cleverly hiding the checkmark (in smallest font) to disallow the uploading of contacts. I have unknowingly let them look through my contacts at least two different times prompting me to never install their app again.
3. Letting others know that you viewed their profile. This is fine as long as you know about this feature but I bet that a bunch of us were taken by surprise in the early days to find out that our "online research" on someone was not private unless if you're incognito.
4. Charging you beyond the free trial period without any email or notification about it. I agree that many businesses do this but some of them like Amazon and Netflix are nice enough to allow you to disable the future payments before the trail ends. Also they tend to notify you about the payments.
5. Letting people have access to my email and other contact info even when I haven't accepted their request to connect. This may have been fixed with the recent change but may just have been one of those hidden features that led to their popularity with the recruiter world in the first place.
I am sure I missed out a bunch of other things here but these are the reasons why I can never trust LinkedIn.