Nxt is considered a second-generation cryptocurrency, and it provides a new platform for a full de-centralized economy.  With all the alternative coins coming out that only change things such as hashing mechanism, time between blocks, starting difficulty, and so on, Nxt brings much much more to the table and was designed this way from the ground up.

It is not an "alt coin" like litecoin, peercoin, and other variants whose code is based on Bitcoin's source code.  It is written from scratch, in all-new source code. The Nxt Source Code was first partially released in January 3d, 2014 and fully released by Jean-Luc, the main developer, in March 1st, 2014. You can evaluate it here.
It provides built-in support for robust features such as a decentralized peer-to-peer exchange, voting system, messaging/chat, decentralized DNS, and options for instant transactions. Many Bitcoin-based currencies implement one or two of these features on top of the existing Bitcoin network. Nxt implements all of them on an improved network model that is built for speed and scale.

It is 100% Proof of Stake (PoS), versus the proof of work (PoW) mechanism the vast majority of other coins are based on.  Instead of using powerful computers to "mine" new coins, Nxt leverages the balances of existing accounts to "forge" blocks, rewarding forgers with transaction fees. The Proof-of-Stake algorithm is efficient enough to run on smartphones and small devices like the Raspberry Pi platform. In addition, this method effectively removes a large security risk inherent in most other coins: the issue of a 51% attack and many of the other vulnerabilities inherent to Proof-of-Work coins are gone. However, currently there's also mining pools available that deliver payout in Nxt if you want to mine, such as Hashrate.org and NxtPool.org.

The energy efficiency of Nxt network makes it a green cryptocurrency, not sharing the negative ecological impact of proof-of-work. Several extensive calculations have been made showing Nxt is, at least, several hundred times more energy and cost efficient than Bitcoin. You can read the Nxt Network Energy and Cost Efficiency Analysis document.

It was announced weeks in advance, unlike mere hours like most coins.  Based on an injection of 21BTC by 73 original stakeholders, 1 billion Nxt coins were ejected from the Nxt genesis block and are currently being distributed to thousands of account holds through giveaways, trades, and bounties for ongoing development of the platform. Nxt is listed on CoinMarketCap and it can be traded in multiple exchanges, such as BTER, DGEX, Cryptsy, Poloniex or Bittrex, among others.

Soon, Nxt will have a very slick feature called "Transparent Forging", which will allow each user's client to automatically determine which server node will generate the next block. This allows clients to send their transactions directly to that node, minimizing transaction times. Immediate and high-priority transactions can be processed for additional fees. An equally important feature of Transparent Forging is an outstanding security feature of the protocol, which can temporarily reduce to zero the forging power of nodes who should generate the next block but do not – penalizing accounts that do not actively support the network, and distributing that power to nodes that do. With transparent forging, even a 90% majority owner of all Nxt will be prevented from branching out and forcing a forked blockchain: if a node has 90% of all Nxt, and doesn't generate a block when scheduled, the system will reduce its forging power to zero temporarily to prevent a bad fork from being forced.