The main problem they bring is how to balance them. You mention several items that were definitely cool back in the day, but some of they also had to be nurfed into oblivion due to the problems they caused. Then on the other side is that a proc sounds cool but to keep it balanced the proc chance is super low or something like that which makes a stat-stick item more consistent and more useful.

That said, the unique item procs idea made a bit of a comeback in Icecrown Citadel and is something we like doing. We have learned a lot of the years on how to balance these types of items while also trying to prevent it from interfering with something like arena PvP. We would like to keep doing procs and we’ll see how things go in Cataclysm.  (Source)

[...] When examples are used that doesn’t mean they are the be-all-end-all reasoning for things. We have also run into problems with procs that are so awesome the item is used way beyond the expected range for it. (Hi Thunderfury, how are you today? Have you been chatting with your buddy Stormherald lately?)

We want the procs to be cool, fun, but not overpowered. The overpowered aspect applies to both PvP and PvE. While we would love to make procs that just ignore the min/max factor of gear, we can’t really do that as it will either make the item overpowered (see above) or possibly so bad that even the previous tier is ahead. Also, while we stayed away from them for awhile, we did make a bunch in ICC and they were pretty successful so will see how things go in the future. (Source)