I agree that any personalization in the letter should be related to the agent's interests and how they relate to your books. If you check this Tumblr, you can find wishlists for a lot of agents:
http://agentandeditorwishlist.tumblr.com/. If they are wishing for something that your book can fulfill, certainly mention that and how your book fulfills that desire.
However, don't mention things unrelated to your book, such as if the agent attended a particular convention (unless you went too and met the agent and had them ask you to send them a query letter.) Ultimately, everything you write in the query letter should show why your book is well-suited to that agent. Personalization is generally good to have, but it should be intentional - don't just put it in to put it in. Make sure it's done to show why your book is suited to that particular agent.