Here's a great essay from Anon136 of this forum that explains the key innovations in Nxt's unique Proof-of-Stake/Transparent Forging implementation. Definitely worth the read.
https://docs.google.com/document/d/1E_ToOMG2l1XThx6YnyXEajXaf6H1k2yjq8XkAF0ScB4/mobilebasic?pli=1I think Nxt could challenge Bitcoin in time. Some of these ideas will be hard for Bitcoin to adapt to (would require changes at the fundamental level, which might necessitate a hard fork - difficult to do when so much money is at stake). Bitcoin could possibly come up with other ways to remain competitive though. I think this year will be very interesting.
eMunie looks very interesting too. As does Zerocoin. From what I gather, Bitcoin could implement Zerocoin, but probably won't in order to cooperate with governments. It wouldn't be able to implement the Nxt or eMunie ideas without a significant rewrite though, as these have very different ways of doing things that are not based on Bitcoin.
Nxt looks cool and all, but I just don't get how it's going to work with the fixed transaction fee. If the value of Nxt rises to, say $1, That means the minimum fee for a transaction is $1! This implies a market cap of only 1 billion. How can Nxt be competitive vs other coins with such a high fee?
Otherwise, the PoS system looks really interesting. Can't we have PoS without fixed fess, or are fixed fees somehow fundamental to the PoS implementation (P2PCoin has the same issue)?