With something as invasive as Google+, it's inevitable that this will have repercussions elsewhere. The only ones I've noticed so far:

• [YouTube]
  • you can't upload videos (existing videos will be deleted when you downgrade)
  • you can't comment
• [Maps]
  • you can't post a review for stores
  • you can't upload photos for stores
• [Play]
  • you can't rate apps, not even from Android devices (wtf?)

Articles about the benefits of disabling your Google+ account:

• carolesthoughtfulspot.com

Articles about Google forcing Google+ onto users:

• Forced Google Plus integration on YouTube backfires, petition hits 112,000