In my experience, 99% of the time, it's because it makes some unmaintained addons not work. Or they simply think issues with it aren't being addressed, despite Mozilla delaying in part to address any issues that are raised.
Once in a while you get someone who thinks that it's creating a "walled garden", but they're people who clearly have no idea what a walled garden is, or don't know how easy it is to bypass the restriction if they really wish to, or how easy it is to get non-questionable addons signed. Those people don't have much of an argument that doesn't boil down to "I don't trust Mozilla enough to let them do this, though I paradoxically trust them enough to run their browser in the first place."
The fundamental problem with mandatory addon signing is that it goes directly against what free and open software is all about. Freedom to use, modify, extend, and share. When Mozilla tells me that I cannot extend Firefox via an addon unless it gets the Mozilla Blessing, the browser is no longer free software. It doesn't matter if there's a special "Exempt Edition" for developers, or if they will currently automatically sign all addons. Their intent is clear and the road to hell has been paved.
To add insult to injury, the reasoning behind this misfeature is asinine. They claim "security" to protect users from malicious addons installed without the user's consent. Just two reasons why this is absurd on the face of it:
* A user with a compromised computer is already compromised. They have much bigger problems than a rogue browser addon.
* Addons are automatically signed by uploading them to AMO.
And to address your points:
* It does make unmaintained addons cease to function. Mozilla worked around this by automatically signing all extensions currently on AMO.
* The feature has been delayed not because of "issues being addressed" but because Mozilla knows it will be a shitstorm which will push even more people away from the browser.
* It is a walled garden -- however the walls are short and don't have razor-wire strung along the top.
Mandatory addon signing goes against every principle Firefox was build under.