As I said in the BitDNS discussion, hard-limiting the network-wide number of registrations will fail. If 50 domains are produced per block, then what happens when more than 50 domains are needed per 10 minutes? Prices will become uncompetitive and the system will lose popularity.
True, the BitDNS must have some sort of load balancing, so that domains remain cheap, but are not spammed.
I would like domains to be like Bitcoin address, You can make as many as you like, but only ones that have coins on them are worth anything.