I've run the Motorhead Events races (2 last year, 5 this year). These are my thoughts...
Log matrices - These are a nice way to mix up the racing lines and difficulty can be adjusted tremendously by changing the spacing between them. The squared and angles are both good to have as they really do act as different obstacles. Easy to make, the center dirt will tend to get dug out though if people ride them like they're crossing individual logs, so some shovel work at the end of the weekend will be needed. Main thing is these need to be bolted together solid.
Log jumps - A log double, even with just 10 foot gap, has a high fun factor, risk element, and some wow factor for spectators.
Big tires - These seem to be getting phased out more and more. They're fun and offer a nice jump. The dirt on the uphill side should be up to the point the tire turns back forward, but any higher than that and the difficulty drops too much and you might as well just have a dirt jump. Landing area is important on this as you'll have those who roll it and those who jump way up and over it. Tires shouldn't be cut in half or smaller than about 4 feet. and need to be secured together very well. There is nothing worse than hitting a soft tire that folds under your bike and moves around. Anything smaller than 4 feet, just doesn't make sense for effort to build vs fun factor and challenge.
The X-games had the rock uphill that looked to be a fun challenge and should be a pretty easy obstacle to build and maintain. Gives different lines for different skill levels as well. Combine that with a two telephone pole step up at the base of the hill with a small preload bump in a couple of the lines about 4 feet before it and you have a good time. Just don't make this directly hill lined up with the starting gate, before the first turn as things will get sketchy very quick it should be at most a fast 2nd or a slow 3rd gear section going into it.
The rock garden turns are a staple, but they really need to be built right to keep the safety factor in check and to keep the rider's wallets, bodies and bikes in a condition to keep coming out there to ride. Big rocks with gaps in them are just begging to snap a leg if someone dabs their foot in the wrong spot.
One obstacle that was horrible was at the Guthrie round of the Motorhead Events race this year. It was a ravine section about 50 feet long with boulders littered throughout. There weren't any good lines through it as the idea was to find a line and put a boulder in the way. This dropped the fun factor immensely and broke a lot of bikes. Not a good way to go about it, but it did give me an idea on how to make a better one. Go ahead with the bowled out ravine stretch and some boulders, but fill it with softball sized rocks leaving at most a foot or boulder sticking through the top. Maybe throw a log or two across it at odd angles. If you can find a way to get moss to grow over all that then you can call it the Missouri Creek section, throw a sprinkler on it for added fun
Also, if you check around for videos of the European series, they tend to have some rather unique concrete obstacles, which would let it all stand out quite a bit.