What is the most important princlple of a nice jig rod reel seat should have?
and what type of reel seat is ideal for jig rod now?
Are there anything to improve it better?

1.the weight?
2.material?Graphite?Aluminum?Is it important with cushion inside reel seat hood,especially aluminum hood?

I think you'll find that modern graphite/nylon seats provide the best feel (warmer than metal) and those with cushioned hood liners hold reels most securely. They're light and corrosion proof.

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For fishing jigs I like the rod to be level balanced or even slightly butt heavy so the tip feels very light and sensitive to pick ups of the bait. So, for a real seat consider if you are always going to use the same reel on the rod and if you are, then the type Tom mentions is my choice. Just make sure you put it at the right place on the blank for the balance you need to fish jigs well. If you might use different reels, I would consider going straight cork (Tennessee style) and tape the reels on.

For my jig rods, usually spinning even for a lot of Bass applications, I'm with Tom on this. They are light weight, comfortable, not terribly cold, give good sensitivity and secure the reel well. I typically go with a "no fore grip" configuration so that a finger can be placed directly on the blank.

I'm building a jig rod on a St. Croix SCV blank and put on a Tennessee Handle with the graphite slide rings on it. I had the rod outside practice casting to test the guide spacings and the handle was very comfortable and did not get cold. The temp was in the teens.

I jig fish for walleye and buy far the best grip I have used is A Graphite Tennessee grip, Period!! If you fish when it's cold get some Hockey tape and give it a quick few wraps and you your all set, much warmer. Use the Graphite arbors when installing so you get the best sensitivity. Mudhole has them as well as Jan's [shop.mudhole.com] If you are jigging with a bait caster try the new Matrix handles.