1) Uptime -- If the uptime of future computers is no better than the uptime of current computers, few if any of us will trust them enough to upload our minds. However, there is reason to suppose that the uptime of future computers will be far superior to that of current computers. Consider, for example, the recent advances in parallel computing hardware, regularly doubling the number of processors available at a particular price point. If a computer is able to spread its work across a large number of redundant processors, it becomes far less likely that a particular computation will fail. Likewise, we're seeing the price of redundant online storage and other computing functions decrease rapidly. As the many aspects of computing continue to decrease in cost and increase in integration, they will be used to achieve uptime approaching that of the massively parallel and redundant human brain. Indeed, in some ways, our computers have already achieved better uptime: they don't need to spend 33% of their time sleeping.
2) Durability -- If the hardware on which our uploaded mind does not endure longer than current hard drives or floppy disks then few if any of us will trust that hardware enough to upload our minds. However, specialized hardware is already capable of enduring many years longer than ordinary hard drives. Moreover, it's not difficult at all to ensure the persistence of data even while the underlying hardware changes. For example, I continue to have fast electronic access to documents that I wrote in elementary school 25 years ago, although the hardware on which they're stored has changed at least a handful of times. This isn't so different than our biological bodies, which depend on the constant renewal of nutrients in order to preserve the proper functioning of our brains.
3) Energy -- If the ratio of computing capacity to energy consumption remains unchanged then scaling computing to degrees required for uploading many minds will prohibit adoption, given current and near term feasible energy sources. However, recent advances in mobile computing have demonstrated that we can increase the amount of computing capacity that we get from a particular amount of energy, and our brain illustrates that a large amount of computation can be done with far less energy than present computing tech would require. Given our track record, it seems reasonable to suppose that we'll continue to increase the amount of energy available to our computers while simultaneously figuring out how to make that energy go farther. There is gigantic financial incentive behind this.
4) Processing -- If the brain is much more powerful than some have estimated then it may take more time for our computers to attain equal capacity. However, assuming our computers continue to exceed Moore's Law, doubling the ratio of complexity to cost in nearly yearly cycles that are themselves accelerating (meta-exponential), even several orders of magnitude in additional computation will be achieved in a matter of decades -- not centuries.
5) Bodies -- If the mind is not associated with a body that properly map to each other then an uploaded mind would be useless. However, uploaded minds would not be independent of bodies. All data has a substrate onto which it is mapped, and the data may be empowered to interact with and control that substrate. Our computers do this constantly, in both subtle and obvious ways, from changing how data is stored to moving mechanical extensions of themselves. At a deeper level, it's just plain important to recognize that minds are meaningless without bodies and their environments. Yet recognition of this does not somehow refute the possibility of changing the substrates on which our minds are operating, which is what is meant in essence by "mind uploading".
6) Politics -- If the politics of access to mind uploading technology get in the way then perhaps many or most of us will not be positioned to benefit from it. However, this is not a new problem, and one that has been overcome with increasing rapidity as the marvels of our technological age have unfolded. Adoption of and access to new technologies does usually start among the relatively rich, but the time between their adoption and that of a more general adoption for emerging technologies has been decreasing. Of course, there is certainly much work to do to ensure broad access to the benefits of technology, but this does not mean mind uploading will never be realized.
Finally, remember that if we or our descendants are likely to compute many worlds like our own then we're likely already living in a computed world -- already mind uploads.
Michael Anissimov and Aubrey de Grey call our attention to Pope Benedict's Holy Saturday address from 3 April of this year. In the address, the Pope presents perspective on immortalism, suggesting that radical extension of life as we currently know it is not a cure for death, but rather a cure for death must "transform our lives from within" and "create a new life within us, truly fit for eternity".
The Pope's message contains some ideas with which I disagree. For example, he questions the value of extending life hundreds of years and suggests it would be condemnation; does he consider us already condemned as a consequence of extending life well beyond the few decades that were available to our ancestors? Perhaps he does, as do many Catholics, embracing a doctrine of original sin and assuming life as we now know it to have no possibility of naturally improving beyond the consequences of that original sin. He reasons that immortalism would leave no room for youth, yet youth is precisely the goal of immortalism -- not merely a perpetuation of geriatric hacks. He also reasons that immortalism would kill capacity for innovation, yet capacity for innovation has only improved as we've extended our lifespans. Finally, he implies that death itself is where we should look to find the beginning of the fullness of life. While I don't consider death the absolute end of identity, I consider it to be among the worst of hollow and meaningless contradictions to equate death with life.
On the other hand, I agree with the Pope's concern that we need to transform ourselves not only outwardly and quantitatively, but also inwardly and qualitatively in all areas of virtue. Without such transformation, his fear of condemnation in immortalism would be realized. Opportunities for human enhancement are not only in areas such as strength and endurance, but also in areas such as compassion, wisdom, joy, enlightenment, courage and the broader esthetic of sublime dignity. From a Christian perspective, the opportunities for human enhancement are modeled in Jesus Christ, glorified quantitatively and qualitatively, physically and spiritually, in immortality and eternal life.
Aubrey asked how we might find "a way to approach them constructively and non-antagonistically." Michael, in understandable frustration with ideas expressed in the Pope's message, complained that we should "not depend on an invisible and silent Stone Age Hebrew deity". In one sense, I agree with Michael. Our perspectives on God should transcend those of our stone age ancestors. To the extent they have not, they are damning us. In another sense, Michael is being too quick tempered, and should give more thought to Aubrey's question.
We will all benefit from seeking greater mutual understanding. Imagine the benefits if we could persuade powerful organizations such as the Catholic Church to support research into radical life extension, if we could help them see that it does not conflict with their faith. Even more, imagine the benefits if we could persuade them to understand radical life extension as fulfillment of Christian prophecy and even as a mandate of Christian discipleship. Far fetched? I'm persuaded that this will eventually happen, given time and effort, at least among a majority of Christians. Our efforts to that end may make a difference in how long it takes.