The things that make an odd jacket different from a suit jacket:
1) Cloth; which we may separate into
There are no hard and fast criteria for any of this. There are certain "know them when you see them" giveaways. Smooth 10 oz clear cut worsted is a suiting, no matter the pattern. Maybe certain shades of blue would be fine for a blazer (though not my first choice), but that's about it. Stripes are traditionally a suiting pattern. Gray -- especially solid gray -- is not seen much for odd jackets. J is right about the button color. Also, I mentioned before that certain pocket and vent configurations were more traditional for odd jackets and considered out of place on suits.
As a general matter, the nubbier, coarser or more wrinkly the cloth, the more suitable it is for an odd jacket. Also, the weave will be more "substantial" and/or have more surface interest. Also, colors can be brighter and more numerous. Certain colors that traditional usage would say are out of bounds for suitings are fine for odd jackets. Patterns can be bolder as well.
Again, there is nothing hard and fast about any of this. Where the lines are, and what crosses them and what does not, are nebulous matters. If you ever have a chance to look through some swatch books, compare the books marked "jacketings" with those denoted as "suitings." The differences should become clear.