Naxx 25 is much closer in difficulty to the old world, 40 man raid. That being said, it is recommended that you be decently geared. Decently is a loose guideline, mainly we're looking for quest blues, maybe a crafted epic or two. I ain't expecting fully pimped out, epic-clad behemoths to clusterfuck Naxx - no one is. What is more important is that you have read up on the strategies and understand them fully, as they are more involving than most of SSC, TK... etc, and often more unforgiving.