Neil Peart.
Rush don't need 'em anymore.
Ya know, I have seen the "This is like Peart leaving Rush" comment made in several places in regards to this, but I think that is ludicrous.
First off, Rush and Dream Theater are very different bands. The three members of Rush are like family; all one needs to do is watch their dinner at the lodge on the extras of the new documentary to see this. There is a camaraderie, a friendship, almost a kinship, that exists between Geddy Lee, Alex Lifeson and Neil Peart. That did not exist with all of the members of Dream Theater when Portnoy was a member of the band. It might have existed between some of them (like JP and Portnoy), but not all of them. It is just a different dynamic. As as such, Peart IS irreplaceable in Rush, just like the other two guys are irreplaceable.
Portnoy is not irreplaceable in Dream Theater. The fact that they are going on without him shows this. They have now changed their lead singer, their keyboard player twice, and now their drummer since the release of their first album. They survived the first three changes, and they will survive this one. Sure, Portnoy is the biggest loss, simply because of everything he did for the band and because of how much his personality was a part of the band, and it will take some time, but they will survive it.