Rome\'s 3 biggest mistakes.

[Tiberius Clodius Corvinus]

3. Christianity. Going to Christianity as its official religion changed the Empire alot. The internal problems caused by Christianity really seemed to sap the Empire at times and leave it more vulnerable.

True on one hand, but apart from the assumption that monotheist religions are more prone to be questioned IMO, I would say that massive internal feud and fighting as well as theological dispute are always signs, that the central powers (e.g. old pagan religion) are already weakened. EDIT: as Demetrius already pointed out ;-) ) (just saw your post)
And regardless of the former, Rome changed christianity a lot vice-versa, and not to the best for centuries to come. :? Just a thought.

That being said, I think there was something of incredible value over there - and quite a lot of it: the mines in the Taunus and Hartz Mountains, and the fertile land along the Lippe, Main, and Rhine. It has always struck me that ancient historians believe the crap about the poverty of Germany (cf. Tacitus, Germania, 2) while their colleagues of Medieval History say that the success of Medieval Germany depended on its agricultural and mineralogical wealth. Control of the east bank of the Rhine was, and is, the power engine of Europe, but ancient historians are strangely unaware of it.

But all this is nothing, if you have to constantly fight for it. I read somewhere, what it took some 20 years in the average to develope a province and as I got it Rome never made a bargain out of Britain. (similiar with the EU today ;-) ) ). And I think Rome was through trading perfectly able to make some profit out of Germany without having to waste its legions there.

So the first Roman major mistake was the change in the policy from controlling territories by treaties and dependency to the direct installation of provinces with all the consequences.

1. The Senate's refussal to create a program and spend the money to properly settle legion veterans. If such a program had be initiated and maintained a large reason for generals gaining such loyalty from their men would have gone away.

This would basically be the second mistake for me. The Roman inability to solve the social question with anything other than "panem et circensem".