Why the hate towards Fassel?

[rypper11]

It's because Mooch plays BacciBall (jk).

There really isn't much. If you stand back and look at both they each have pros and cons. The difference lies in when they turned up as potential replacements for Gibbs. At first, we all assumed it would be GW. Then Fassell entered the picture and he was seen as the bad guy who was keeping GW from becoming coach. Now, Mooch is the anti-Fassel.
Seems dumb, I know, but it goes to "the enemy of my enemy is my friend."
Personally, I think Fassel is tainted by Baltimore and I'd rather not have another former NFC East foe coaching the Skins (Norv?).