The things people are putting on their "best" lists are definately worthwhile, but very few of them need to be nerfed:
Unbreakable Boots (recommend reduce mana amount)
Capture Lock (as mentioned in other threads, give lock immunity to a flag for several seconds after a capture lock wears off)
The "blink"-ability general favor item (give it a cast time of 1 to 1.5 seconds so that it's interruptible, but farther distance to make up for that so that it's more like a mini-teleport)
Blade of the Serpent (reduce damage to 20)
Blood of the Fallen (reduce health a bit, or reduce it a lot but up the regen)
Narmoth Ring (reduce health slightly)
Heaven's Wrath (reduce small-unit knockback (instakill) radius so it's not quite as good for killing masses of creeps; do not nerf cooldown or anything else)
The following need buffs (imo)
Most Favor items (the ones with the weaker abilities need some small stats boosts; the abilities themselves I wouldn't mess with)
All +% mana regen items, since those basically suck without 15+ (Vlemish/Hungerling helms or mana per sec flags) mps already (recommend just get rid of that stat and give those helms different stats, like give one 5 health/sec and 10 mana/sec)
Angels (don't have the usefulness of priests or cats, and don't have real combat-viability like Giants, so it's pretty much always bought right before cats since you don't want to spend 3k to give the other team experience; give them more damage)
Trebuchet (useless on most maps, with the exception of the fortress game mode; need more forts to pop up via warscore in most maps)
Finger of God (it's not too horrible, but when you compare it to Giants, it sucks balls; shorten cooldown to 20-ish seconds)
Building Damage (damage doesn't scale to endgame-acceptability, especially without a way to revive or plant new towers)
Building Health (recommend give %damage reduction (or at least armor) instead of health since the health component is fairly useless, keep hp regen)
Reinforcement Damage and Health (values are okay, just reduce costs a bit)
Currency (gets too expensive, since by the time you can afford 2 & 3 they're not worth that much for an extra 2 gold/sec because there's not as long left in the game)
Death Penalty (needs to be much cheaper, or more like 20% reduction per level)
Rework most on-hit items, since most of them are too expensive given their unreliability.
A few of the consumables need to be buffed, have their effects worked into trinkets/favor items, or have a price drop.
As an unspecific request, I'd like to see either some of the existing items (including favor items) or some new items focused more heavily around health/mana regen (even dual regen) than the large +health/mana boosts that always accompany them.
That's all that's come to mind at the moment.