Some touring bikes have steel (Chrome Moly) frames, as do our Surlys, the Trek 520, Fuji Touring, and others. The steel frame has a bit more give than aluminum, which means an all-day ride that’s gentler on the body. Threads on the frame are less prone to stripping, paint adheres better (our empirical Surly/Devinci observation) and if a crack begins to develop while you’re on the road in Uzbekistan, there will be more repair options. (You can tell by looking at the welds whether a frame whether is steel or aluminum: aluminum welds look like quite a wide bead of goop in the crack, whereas a weld on a steel frame is barely noticeable.) On the downside, the bike will be a little heavier (for us: 13.4kg (Surly) versus 13.0kg (Devinci)), but since you will likely be carrying 20+kg of gear, you won’t notice the minor difference.
Some travellers go for a bit of front suspension if they’ll be doing really rough roads. For most of our travels till now, we think suspension would just add weight, be one more thing to go wrong, and the up/down movement permitted saps energy from forward propulsion. Moreover, suspension limits the choice of racks. For all but the the lightest credit-card touring you’ll want front panniers to distribute some of the weight forward, so make sure any bike you consider has braze-ons (reinforced threaded holes) on the front forks for easy mounting of a front rack.