Like, seriously, there has not be one wasted second in any episode. And, even though it nearly completely lacks anything resembling a plot beyond "Undead things appear, people defeat them" it's never boring at all.
It manages to make fun of everything - from gun nuts, to traditional families, to racism to sexism to slasher films to Buffy.
And even though the main character is a complete asshole you can't hate him because every part of the show is so ridiculously good natured.
It also managed to give us Naked Zombie Lucy Lawless without making it seem exploitative.
Making it only 30 minutes long was genius - because if it was an hour long show the formula simply wouldn't work. At its core Ash Vs. Evil Dead is a weekly Saturday Night Live skit, and like those skits, being short works in its favor.
I think the best part of it is that every actor shows that they are totally in on the joke. The winks and nods keep things at just the right level of silly.
This is Buffy, if Buffy had been written by Cheech and Chong.
Quite simply it is the best television show ever put on the air.