In summer 2012, I wrote a three-part series of editorials, detailing everything that made Hogwarts such an appalling school. Because the truth of the matter is, that while Umbridge is pure evil, some of her ideas were not bad ones (even if they were executed rather poorly).
Part 1: Dumbledore's Giant Mistake lists all the reasons why Hagrid is my least favorite character in the series, and why he is the singularly WORST teacher in all of Hogwarts.
Part 2: The Other Professors tackles all of the other dreadful professors Dumbledore has hired based on everything except pedagogical merit. Trelawney, Firenze, Slughorn, and even Filch all come under scrutiny. (The DADA professors are excepted because of the limited applicant pool, and Snape is tackled under the Snape essays).
Part 3: Punishment and Safety looks beyond the professors at the myriad other problems at Hogwarts, focusing on the wildly inconsistent discipline and wildly ineffectual security.