Why can't Bitcoin use a very large blocksize such as 1GB? In the current blocksize limit debate multiple proposals have been made. Bitcoin XT implements an increase to 8MB right away plus a doubling every two years. A competing approach used 20MB.

What specifically would make it a bad idea to go with a very high limit such as 1GB as opposed to increasing to a smaller limit such as 8MB?
If you made the block size that large, the rate of network divergence would increase drastically as more and more miners would find blocks while still transferring a block that someone else found. (You have to consider the network latency of propagating 1GB to peers every 10 minutes).

There are other problems. Large blocks make it harder to do full verification of transactions, meaning that it would be done less. That would mean more and more miners who trust others to verify transactions on their behalf. This could threaten Bitcoin's decentralization.

There are lots of other problems, these are just the most severe and obvious.