The biggest rejection-issue I've seen for all longer weapons is "It's too whippy". That is, the deflection angle is too great over the length of the weapon. How much is "too great" and what the proper "deflection angle" might be is totally subjective and up to the local Weapon's Marshall. While the book does say weapons should "not bend more than 6 inches from true when a moderate weight is applied to the tip and the grip is held level" (ARB pg. 82) it's still a judgement call about by the Marshall. So, while I can offer insight into what I've done and the weapon's I've had pass, your best bet is always to speak with your local Staff and get the carrot direct from the horse (or another, more appropriate metaphor).
"Whippiness" is almost always a function of the weapon core. The easiest way to almost ensure your max-length 1-H spear is accepted is to use 3/4" PVC pipe. This is the gold standard from which the original weapons were made and is almost universally accepted as kosher. It does, however, make you a brick of a weapon. 3/4" Schedule 40 (thin-wall) PVC or CPVC is a good remedy for the weight problem as it's the same outer diamter (three-quarter an inch) but a slightly larger inner diameter, making the wall thinner and the pipe lighter. I have found that SCH-40 does not greatly increase the whippiness but greatly decreases the weight. It's a good trade-off. SCH 40 is harder to find as it's not a standard weight PVC. You can either call around to Hardware stores in the area (dedicated Plumbing suppliers are a better bet) or get it off the Intertubes, if you don't mind paying S/H. There are, of course, more expensive cores like kite-spar, carbon tubing, or something more exotic (I've heard of, but never seen, bamboo coores), but that might be a heavy investment for your first weapon: PVC caps out at $2 for 10 feet; carbon turbing is around $8 for 6 feet.
Final note, if you do use 3/4" PVC make sure to find insulation built for a 3/4" pipe - trying to jerry-rig a smaller diameter foam is not only uneccesary but dangerous. When you're buying pipe-insulation, just keep a weather eye out for the Inner Diameter vs the Outer Diameter measurement: you want insulation which fits 3/4", not foam with an Outer diameter of 3/4".