Nasus is relatively weak at the moment due to several factors besides just items too.
Gameplay factors include the speed of games, which are faster, and the the fact that teamfights are becoming more important. Crowd control comps are also becoming more prevalent and tenacity is pretty crummy at the moment. As Lazarus mentioned a lot of items Nasus builds received nerfs and the frequency of items such as
and
being built are making it harder for him to group at all.
As a Nasus main, I have to play pretty hard (harder than my opposing top laner) to MAKE SURE I hit my power spike. My goals in laning phase are the following:
Avoid any early death
Get stacks, but ALSO keep up in CS against my laner; Nasus can't deny early farm and zone like other champions
Exert some type of map pressure with 1 TP play OR first tower blood
After laning, I tend to simply go where the towers are on the field. I probably group more than other Nasus players, but I hate grouping unless I am very strong.
Almost every top laner is better than Nasus in the early game, and with the current state of the game, almost every top laner is more revelant to successful gameplay than Nasus.
Nasus' kit is fine. The real problem is that several items he had a tendency to build were nerfed, and he receives very little reward for stacking well and being patience.
Riot can help Nasus in two ways: (1) Give him slight buffs or (2) Buff some tank items that he builds.
I think buffing tank items could be problematic for the overall game, and I think giving Nasus minor buffs will go along way with his useability and win rate.