I played Nasus for a long time, his Q may be OP, but everything else about him is UP (except his ult which is neither).
Spirit Fire is damn near useless until level 4. The cast speed is so slow, it's like saying "Don't stand in this circle for the next 5 seconds." It's also one of Riot's brilliant ideas to have a spell do magic damage and reduce armor. Lolwut?
Wither is the best slow in the game, but it's only that, a slow. Now with more Tenacity items, it's effectiveness is really poor. It slows more during it's duration, the most potent part is at the end. Since Tenacity cuts durations 25-35%, you're losing it's most effective part. Leveling it up only gives you more duration, no CDR or mana cost change. The latter is especially bad because 100 mana at level 1 is often 1/3rd of your mana pool. It doesn't sap AS/MS and it isn't a hard CC like stun/silence.
Q is really good, but unlike Veigar, it's a melee skill. So it's much harder to farm if you're zoned out.