Need to clear some issues.

Quote Originally Posted by Watchman
Back in the 7th century or so the Arabs had besieged the place for years, to no avail (I think the Emperor eventually managed to offer them enough tribute to get them call it off). And they were still running on the high gear of the newly established faith's militant expansionism phase. The as-such usefully unifying religious fervor didn't much keep them from having serious issues with Sassanid war elephants, however, and routing from several battles for other reasons. Ditto for the Moors when they butted their heads bloody against Frankish lines at Poitiers, 732 AD.
poitiers was a case of a small arab force against a much larger frankish force, they were already retreating from their recent raids when the franks organised for a defence.

The important factor was the frankish shield wall as most raiders were cavalry, think of it as a phalanx, the arabs didn't really put much of a fight though, most were too concerned with the loot they gained.

On the sassanid war elephants, they did adapt eventually, very remarkably fast too, they used camels and elephant slayers in a certain battle, the elephants themselves pretty much did the work for the arabs in routing the sassanids.

But as for the franks, the arabs didn't really adapt becuase contact with them before the crusades was pretty minimal, thats why at first crusaders won against superior odds.

Quote Originally Posted by Watchman
The Muslim (initially really Arab) ability to overrun a poerful, long established empire and bring another to its knees in their initial Blitzkrieg came from many reasons, but among the chief ones was the simple fact Byzantium and the Sassanids had been fighting each other more or less intensively for quite a while and were caught in a rather weakened state. The Arabs may not have been steppe nomads, but quite a few of them were desert nomads and hence capable of considerable strategic mobility, especially in deserts, that the imperial armies found difficult to match.
You forgot that the both empires were fully capable of repelling the arabs, the real problem was the arabs had very good to excellent commanders, even if the empires weren't weak, at best the entire qaddisya and yarmuk campaigns would have been longer, due to the lack of competent sassanid and byzantine commanders.

Now if Heraclius wasn't totally nutters, that would have been a different story.

Quote Originally Posted by Watchman
Around those times pikes had fallen from use (although to my knowledge standard one-handed infantry fighting spears the world over have tended to reach lenghts of some two and half meters that doesn't quite compare to the up to 5.5 meter Hellenic and Medieval pike...) anyway, and the Arabs swiftly copied the cataphract principle to the extent their resources allowed - which in practice meant most of the horses had to make do with hardened leather barding, for example. Well, at least that kept the load down and retained greater mobility.
I don't believe the arabs copied armies they met, they just recruited on the move, visigoths, sassanids, byzantines, and whoever they met on their way, by the time they reached the pyrannese alot of their army were composed of berbers and visigoths, and at Talas it was turkic.

Arab armies were relatively light at the time of conquests, chainmail being the heaviest armor they had, and only available to whoever can afford it (usually nobility).

Anyways, i agree mostly with kraxis, it shouldn't be overtly exaggerated, just make the muslims competent at least without the need to micromanage alot(especially in MP, muslim faction players know how hard it is controlling horse archers and micromanaging them), and also provide units for later ages, in MTW they were stuck with very few mid-late game units and had to rely on mostly crap units compared to the chivalrics and heavier cavalry the europeans got (even the byzantines suffer from this to some extent).