As the other guys have said, the diff is a problem, but not just the hemisphere, the half shafts shear, the planet gears split, pins wear through, thrust faces disappear.
I'd actually suggest a Quaife Diff, lifetime guarantee as long as you don't exceed 750 HP 
, besides, if you're going beyond about 90 HPs, you'll want / need something better than an Open Diff.
The Pinions can split and / or loose teeth, however, getting a good, on spec case helps here. We've enven had a few Crown Wheels loos the odd tooth too. The OEM CW & Ps were actually quite good, just need to take to them with a die grinder to remove any stress raisers, some of the after-market ones are good and some are not, like everything, it's a bit of a mine field.
In the way of production Gearbox Cases, I've found the earier DAM 5626 Cases amoung the strongest, but even then, you need to go through a few of them to get the best, the machining tallerances weren't great on these and it seemed they got worse as they went on. Without the right gear, it is difficult to measure the cases and even with the right gear it takes about 1 - 2 hours to properly measure up a case.
The Primary Gear Bushes can give trouble, but a lot of that comes down to how it's driven, ie, avoid slipping the clutch with lots of Ponies on the loose.
Early Idler Gears Bearings are way too small however while the A+ type are fine, however because the bearings are so much bigger, the thrusts take a real hiding - robbing Peter to pay Paul!
In many ways, straight cut drop gears maybe the go, if you can put up with the noise, but don't overlook helicals, they are stronger than straights,
With the A+ Laygear, the Layshafts seem to wear on the Big End as they only have a single bearing here, where as the pre A+ ones had two. I have had them internally ground to fit a second bearing in there and that elimiates that problem. Swifttune and Mini Speed do high quality layshafts.
When buying bearings, have a good look in particular at the needle rollers (all of them), many I've found these days have needles in them about 70 - 80% the length of the originals, so less support. You'll probably have to try a few suppliers before finding what you're after.
I loctite the Mainshaft Bearing in place, as next to the diff, the second weakest point of the gearbox is the main web, even in a DAM5626. Long ago (before the 5626's were invented), we used to modify the stock cases but adding more metal to the web, then machining the tunnel true again, as well as filling in that silly bolt hole where they used to crack from. KAD do a Mainshaft Support, which takes the place of the small end cover (that the speedo drive goes in to), these are a good idea and will help, but the Main Shafts are not strong enough at this point to take full advanage from them.
That's about all I can think of off the top of my head, but I know there's more!
It's not hard to spend as much or more on making a good gearbox as it will the engine.
