I'm not hugely experienced - I have done one big trip around the Alps in a group and a few laning trips and off-rading and winching training courses, but I have a very clear opinion on this. Tyres.
The tread pattern and size of your tires will have more influence than any other aspect of the vehicle, and probably more effect than ALL other aspects of the vehicle combined. High engine performance, tall suspension, fancy dampers, bullet proof transmissions and lockers will be of no use at all if you have no grip. As commented by others, they lift the axles too, not just the chassis and body. They are also the easiest and cheapest upgrade.
Lockers will only help if the tyres have enough grip on one or more corners to pull against the resistance of the vehicle, and if you're dragging a heavy vehicle through deep mud or over large boulders or stumps, you're going nowhere. Lockers are expensive, a significant job to fit and can have nasty knock-on effects on the handling and on the longevity of other transmission parts. They also tend to be more of a problem for the expedition drive than an asset - they won't get you through a badly selected route; they'll just get you stuck in a harder position to recover the vehicle from.
Suspension is an easy one - expedition vehicles need heavy duty suspension for the increased weight, but extra height is your enemy. Competition vehicles need softer suspension and longer articulation is advantageous, but again, not as much as grip from good tyres - you can clear a section with three good patches of grip, but you won't clear one with four patches of no grip. Suspension lifts are an accessory for the poser, not for the offroader.
When choosing tyre patterns, know the terrain you will be driving. ATs are great for most surfaces for expeditions, and BFG ATs were the choice for all byt two of the Alpine group I was in and did the best on the roads, dirt tracks and loose shale. They're not too bad in most sand or in moderate snow, either. They do struggle in thick mud or on wet grass, for which MTs are better, but MTs are not pleasant on wet roads and struggle more on shale and wood (bridges, play site see-saws, etc) and dig you in in sand. Extreme tyres will be brilliant in deep mud and on rocks, but truly horrible on roads (wet or dry), shale or sand. For sand driving, you want something very mild to avoid digging in.
You also need to consider tyre dimensions. A lot of people go for the tallest, fattest tyres they can find. Like the suspension lifters, it's a path that leads to a less capable vehicle that just looks aggressive to the uninformed. There's a good reason that LR stuck to 7.50s on the Camel Trophy and the MoD still use them on their Defender fleet - their height gives a good balance between ground clearance and driving torque, clearance of the wheelarches, steering lock and resistance to being pulled off the rim, while their width enables them to cut through the crap on the surface of wet terrain and find grip underneath; wide tryes float above, giving worse grip in mud and are more prone to aquaplaning on wet roads. The only places wide tyres are beneficial are where you would also reduce pressures to increase the footprint, ie on sand and snow. WMIK and desrt patrol Defenders in Afghanistan are using bigger tyres than 7.50s, but that's because of the very heavy nature of those particular Defenders and the sandy conditions. Have a look at the tyres on WWI trucks and see how they coped with the mud in Flanders, the Somme and so on to see what I'm getting at.
