I would like to preface this post by saying I have played Nasus a great deal. Not as much as another champion or two, but I have certainly played many games with him and tried all variety of builds. At the current time, I play him as something of a tank. I've found other builds to be too frustrating with concern for his squishiness.

I would also like to say I have yet to see another Nasus played well. In fact, I am the only Nasus I have seen in the past few weeks with a positive score at the end of any game (on the rare occassion he is chosen, these days.)

For a while, doing well with Nasus (typically for a reason of little or no deaths) was a point of pride, as all of my friends agreed (having tried him themselves) that he was rather weak. However, in more competitive games I worry that choosing him is severely crippling for an individual player. I have some tweaks which I feel are modest and would improve him without making him overpowered.

1) Allow him to fill his archtype:
Nasus is defined as a tank. Unfortunately, his base stats do not support this. First of all, let's look at his base HP at level 18: 1940. This is, on average, 300-400 lower than other champions listed as tanks. (Blitzcrank: 2200, Nunu: 2300, Alistar: 2400, Kayle: 2100, Warwick: 2100, Tryndamere: 2130) The only tank with lower HP is Amumu, however Amumu is not a concern for comparison, as he is unanimously considered one of the best champions in the game due to his excellent spell damage and control potential.
What's further concerning is Nasus' HP is more comperable to much higher damage characters who are -not- defined as tanks. (Master Yi: 1906, Jax: 1885, Gangplank: 1880)
My suggestion is to slightly increase his base HP. He currently starts with only 410, which is abnormally low. I would propose let him begin around 425-435ish and change his HP per level from +90 to +95.

2) Spirit Fire and Wither
These abilities almost seem like they were "swapped." Spirit Fire's damage, needless to say, is very poor. This is okay, however, because its armor debuff is its primary function. However, it is also easy to avoid and does not apply a debuff that remains on the target for a brief period after exiting the circle. There is no punishment for failing to escape Spirit Fire's delayed explosion.
Wither is currently probably the spell which frustrates me most. Before you pump considerable points into it, the first few seconds of the snare are intensely underwhelming. If you choose to use Wither in a melee fight to actually gain the advantage of the attack speed debuff, the enemy champion will simply run away as it slowly becomes apparent it is losing. Knowing this, most people use Wither to chase, because otherwise you will never get a kill as Nasus. In this situation, unfortunately, the attack speed reduction has no impact.
My suggestion? Switch the debuffs. Wither should reduce armor, most likely starting at an amount slightly lower than spirit fire, and concluding at an amount slightly higher. Consequently, spirit fire should reduce attack speed, as the only champions who would remain in it are those actually trying to attack you. If it were to gain a debuff with a minimum duration, that would also be a reasonable change. Perhaps 3 seconds, but potentially after increasing the delay on spirit fire.

Finally, reduce the cooldown on wither very slightly. Perhaps from 15 seconds to 12. It currently has the same cooldown as AoE snares of considerably more effectiveness (Mocking shout, Mega Adhesive) rather than the cooldown of most other single target snares (Debilitating Poison, Seismic Shard, Reckoning) which, in my opinion, are considerably more effective.

3) Mana Regeneration
Nasus has very low base mana, and is forced to farm units with Siphoning Strike to be on par with other champions. At this rate, mid game each creep wave will cost you 80-120 mana assuming you are landing an SS each time it is available. There are two possible solutions to this:
a)Soul Eater also restores a very very small percentage of the damage you do as mana. Something like 2/2.5/3% for the respective ranks.
b) Successful last hits with Siphoning Strike refund 20-30 mana. Not 100%, but some reasonable fraction of the spell's cost. Farming with Spirit Fire is costly enough for a champion with so small a mana pool.

Minor Changes:
1) A display for the damage gained by Siphoning Strike.
2) A black swirly ring around Nasus when he uses his ult, indicating the range of the life drain.

TLDR
To sum up these changes, here is a summary of the patch notes I would like to see concerning this champion.

Soul Eater: Now restores 2/2.5/3% of damage dealt as mana.
or
Siphoning Strike: Successful killing blows with this ability refund 75% of the mana cost.

Wither: Cooldown reduced from 15s to 12s. No longer reduces attack speed, instead reduces armor by 10 and an additional 2/4/6/8 each second for 5 seconds.

Spirit Fire: Delay increased by 50%. No longer reduces armor, now reduces attack speed by 40/50/60/70/80% for 3 seconds. Debuff is re-applied each second the champion remains inside Spirit Fire.

Base health increased from 410 + 90 per level to 435 + 95 per level.

I hope nobody finds these suggestions too outrageous, and I appreciate any criticism. Thanks for reading. (:

These changes would not reduce Spirit Fire's farming effectiveness, and you could obtain the same results with Wither in these circumstances. More effectively, even, as they would not simply step out of it before you could do much.

Amumu isn't the only tank with lower HP, Rammus has a lot less health and his health and mana regen are a lot worse as well. You could say defensive curl kind of compensates for that, but it's not like Nasus doesn't have a skill that gives him lots of health.