This is not your typical Nasus build. But this build will allow you to carry games most of the time. Lifesteal Quints makes your sustain in lane hard to handle. There are few champs who can put out the harass to keep you from farming. With quints at lvl 1 you have 20% lifesteal.

Pro's
Heavy damage especially to squishy targets. Can take almost all of their health in 1-2 hits
Heavy sustain from lifesteal items and passive. Laning phase is easy to just farm q.
With trinity move speed buff and wither no one gets away.
Early game damage and lifesteal makes it hard to counter.
Most ppl wont focus Nasus in team fights. They go for the squishies leaving you free to destroy the back line. And if they unleash everything on you, your carries can sit back and tare them down.
Building like this removes nasus' counters. Only tryndamere gives this Nasus any problems and thats only early on.

Con's
Not near as tanky as you can be with nasus.
Lifesteal only works when you hit something so heavy cc can be a problem.
If you are not able to get fed (it can be really easy with a q that hits for 700-800 dmg mid game) you wont be as heavy as a threat.
Outside of Ghost and wither no real escape.

When farming last hit with Siphoning Strike as much as humanly possible. If your good then 275-300 q farm by 20 minutes should happen most of the time. The laning phase isnt about killing the other guy for nasus. Its about farming your Q so that the damage output is crazy. The higher the damage the more you get back from the lifesteal. So just play defensive until you get a trinity force and then if you want you can stop your farm briefly to get a kill or force them out of lane. If you farm good there are no counters. Rush a sheen as early as you can. That makes farming your q easy as it gets.

This is a high damage high sustain build that can go tanky mid to late game as needed. Try it out for yourself before you judge the build. I usually have 10 plus kills and i have had 5 quardras and a penta building like this lately. I have only been doing this since the changes to Trinity force.