Some games that I own but don't like playing I decided to hide. But I failed to find a way to show all hidden games. The only way I found out to see the hidden entries is search for the game name in my collection. Is there a button I can click to show all hidden games at once?

There sure is!

On your My Collection screen, if you click on the "advanced filters" text, it will give you several additional options for filtering your collection. One of which is defaulted to "Games Not Hidden" that you can change to either just show "Games Hidden" or both hidden and visible games "All Games"

Thank you dhobo. I found it now.

Another question: is there a way to quickly find threads you participated in?

Just now, I just went to look if someone answered by going through the list of topics to find this topic I created, but sure there must be an easier way (in case some time has passed since checking the forum and threads I participated in get buried for instance)?

If you look at your profile page, towards the bottom, you'll see the 5 most recent threads you've posted in. Hope that helps!