Since Megadeth have been performing Rust In Peace from start to finish on their "Rust In Peace 20th Anniversary Show", their live set probably won’t offer too many surprises. However, Slayer have never played Seasons In The Abyss before, meaning that songs like "Spirit in Black" and "Temptation", which have never been played live, will get their debuts, two decades after they were first written. It's the second time Slayer have played a full album before; the last time they did was to celebrate the 20th anniversary of 1986's Reign In Blood. The live shows from that tour culminated in the band being drenched with fake blood during "Reign In Blood", leaving the door open for a similar thematic element to their American Carnage shows.
The only downside to setlists like these is that it won't please everybody. Slayer and Megadeth have such extensive bodies of work that to devote anywhere between 60% to 75% of the show to one single album means that a lot of songs have to get booted. Considering that both bands have recent albums out (Slayer's World Painted Blood and Megadeth's Endgame, both 2009), there's a chance that many songs, both new and old, will be shelved. However, an opportunity to see the original Slayer lineup playing Seasons In The Abyss, and to see Megadeth with Dave Ellefson again playing Rust In Peace, will probably reduce any complaints to minor, forgettable quibbles.
The Canadian leg of the tour commences July 23rd in Quebec, with the show hitting American soil on August 11th in New York, and concluding on September 4th in Oregon. The promotional video below shows the schedule for the original dates (before their postponement).

