Ridley's Cohort said:
Even if your Wizard is hosed, he is not actually dead. Seems comparable in its nastiness to Slay Living and Baleful Polymorph IMO.
Slay Living is Touch.
Baleful Polymorph is Close.
Feeblemind is Medium.
SL kills.
BP for the most part prevents most standard actions (except for Psionic types and Druids with the Natural Shape feat). But, BPed targets can at least think, run away, hide, etc. And, Dispel Magic or Break Enchantment can negate it.
Feeblemind prevents most reasonable standard actions except normal melee fighting. For example, an Int 1 (low animal int) creature should be unable to use complex tools like a crossbow or a bow, so the enfeebled Wizard can fight with a dagger or staff maybe. Beyond that, he is basically a drooling idiot. And, it takes higher level spells to negate.
The -4 to saves is not really a big deal. Mostly what it does is switch the saving throw from a good category for Wizards and Sorcerers to a bad category (like Slay Living and Baleful Polymorph already are).
So, the real issue here is range. Feeblemind is somewhat stronger than Baleful Polymorph as is and it is longer range too.
Sure, the BPed target cannot fight, but it might be able to overcome its own problem, even if its allies get slain. The Feebleminded target is pretty much SOL if his allies cannot assist.
When compared to Slay Living, the issue becomes one of touch attacks. There are defenses against those such as miss chance spells or Death Ward or even the standard difficulty in getting real close to a Wizard or Sorcerer that Feeblemind does not have (e.g. Levitate or Fly will typically prevent it). The real protection against Feeblemind does not occur until 8th level with Mind Blank. Before that, there are few defenses against it except maybe Protection From Evil (which might not really help).
All in all, Feeblemind is a bit potent for 5th level compared to the other basic save or die type spells.