Emperor Thaurissian is a 5/5 for 6 with a crazy ability.
Chromaggus is a 6/8 for 8 with a crazy ability.
Nefarian is a 8/8 for 9 with a pretty underwhelming ability.
They are doing cool things with Legendaries, except where it really matters. Nefarian's effect is really cool and interesting and flavorful, but nearly useless when the body is a 8/8 for 9mana. Its draw 2 unlikely to be useful spells. Such a bummer.
Chromaggus and Thaurissian on the other hand are amazing. The ability to draw 2 copies of every card for a couple turns is exactly the kind of powerful effect that would make a 8 mana card worth it in a deck and as a bonus the guy doesn't get destroyed by BGH. Thaurissian can be a really powerful efftect (assuming it stacks) if you can keep him alive for a couple turns your probably win outright with cheap cards being impossible to stop. Super cool idea. I hope the other cards are as interesting.