I want to contribute an alternate viewpoint from the post by Jamie:
<<Also look for a sword with the balance point within a couple of inches from the hilt.>>
Many individuals seem to prefer swords that balance (POB) closer to the guard than is historically accurate, and this close POB is influenced even further by sport wushu where fast and flashy moves require a POB very close to the guard (resulting in light weight "whippy" blades). Jian made for actual use (as opposed to ceremonial/decorative) from the Qing Dynasty and early Republic typically have a POB about 4-6" beyond the guard depending on sword length (you can search the Sword Forum for this info). While a closer POB often seems easier to control in relatively slow speed solo work, it actually seems to interfere with proper applications against an opponent at moderate or full speeds.
If you are simply looking for something that is comfortable to use in solo practice, then a closer POB and a lighter jian may be acceptable. If you are looking for more accuracy in how the applications would be applied against an opponent, then I would recommend getting a jian with a more historically accurate POB and a weight around 1 ½ to 2 pounds. While this may initially seem uncomfortable to hold, if you use your body to properly control the sword rather than solely the arm and wrist, then it should not be too stressful, and consistent use will quickly condition the arm and wrist.
As I experience it, some of the advantages of historically accurate balance and weight are the following:
Tip Control/Effectiveness: The tip is often used in Taijijian, and for these techniques to be effective (especially the pokes) the tip must have enough weight. The weight (and POB) also seems to help keep the tip on target, a lighter tip being easier to inadvertently jiggle off line by a hand/wrist movement, as well as being easier for an opponent to deflect off target. The combination of historically accurate weight and POB seems to be ideal for such tip techniques.
Lively Blocks: Taijijian blocks emphasize deflections that keep the tip simultaneously threatening the opponent. For example, simple left or right blocks typically move the jian handle to the sides while keeping the tip towards the opponent. These blocks seem to be facilitated when the POB is closer to the pivot point of the block. Also, techniques where the jian pivots around the opponent's weapon (e.g. jian outside then pivoting around to the inside to block or set up an attack) also seem to benefit from the more historic POB closer to the pivot point.
Chops: Although perhaps not used particularly frequently, POB and more weight farther out on the length of the blade facilitates the chop technique (as is clearly illustrated by examining the design of the familiar ox-tail dao, which emphasizes chopping and sweeping techniques more than the jian).
While the above seem to benefit from historic weight and balance, I also do not notice much detriment to other techniques (thrusts, cuts/slices, etc.). Even speed and control, which one perhaps would think would improve with a lighter jian and a POB closer to the hand, do not seem to me to be significantly interfered with (assuming one is using the body to properly control the jian and that the flashy wushu style movements are not being performed) when using a more historic accurate jian. To me the speed and control of the above mentioned techniques seem improved (although I don't have any empirical evidence to support this claim) when using a historic sword (I have one that is probably from around the mid-1800's, as well as wood ones made to historic weight and balance for use in sparring practice).