I can't believe we've got another thread on this issue because it's common sense. My sig was made for moments like these.
What? The most straight forward explanation regarding the Narada on why it looks so different compared to ANY ship we've seen in Star Trek, Romulan or otherwise, is explained in Star Trek Countdown. It was modified with Romulan Influenced Borg technology that for all intents and purposes turned the ship into a massive mesh of parts that have no clear design guidance.
It looks similar to the Romulan drone ships in ENT:
http://ent.trekcore.com/gallery/albums/4x14/aenar_300.jpg
http://ent.trekcore.com/gallery/albums/4x14/aenar_301.jpg
http://ent.trekcore.com/gallery/albums/4x14/aenar_446.jpg
No Borg tech necessary.