I worked at an R&D firm that focused on natural fibre composites until about a year ago, and we had some projects with nanocellulose going on while I was around. Apparently it's pretty tiny stuff (maybe that explains the nano part, hmmm...) so barring a huge breakthrough it's not going to be threatenning kevlar fabric anytime soon since weaving it wouldnt be practical at this stage of the game. If it's used in composites, it'll be as a reinforcing fibre that gets mixed in with the matrix (some sort of polymer) which will give it a rigid shape.
The article explained the challenges fairly well. First, it's hydrophilic, so bonding it with a hydrophobic polymer won't work very well and you'll end up with poor fibre/matrix bonding which is currently the achillis heal of most natural fibre composites. Sure, there are surface treatments that can help this, but most of the current ones degrade the performance of the fibre while improving the fibre/matrix bond so that the strength increase isn't all that great (I'm talking about macroscopic natural fibres now. this may not be the same for nanocellulose but I expect that it's similar since the chemistry is similar).
Anyways, best of luck to all the people trying to make this work. I'm making plans for a transparent aluminum boat, but I'd love to use transparent wood if possible :)