I thought about this and if NNF was database powered I would have thrown it in instantly, but the way NNF saves its data – straight to RSS – I wonder if this would have any noticeable performance impacts?
Implementing a view counter would add an expansive file lock, read, and write chain every time a thread is viewed. All of these have to be done in order. So if a 100 people visit a popular thread the server will be locking, reading, and writing, the view counter a hundred times in a row. As a popular thread will probably have many replies we can assume the RSS file to be big, which slows downs this process even more.
So now the server has 100 view count updates queued, then what? Then any new replies will be added to the end of the queue. You will hit reply and then your browser will just sit there, waiting, you might even think the website has gone unresponsive or is broken overall.
On a good hosting platform and with forums the side of this one here these problems will probably be inexistent. But they are something to think about.