Add too much other stuff and you overwhelm the taste of the tequila rather than complementing it.
That's my conclusion as well.
Not a fan of other liquors or flavorings (such as cointreu) aside from lime and agave in my margs. Just the tequila.
The nice thing about agave syrup is it's easier to control the amount of sweetness with it. It's more mild than sugar. I'm also not a fan of too-sweet margaritas; either a nice balance of sweet and sour or, if anything, lean a bit more toward the lime.
Damn, all this talk has been making me thirsty for a good margarita, but I'm trying not to drink during the week. Must divert my attention away from this thread...