soft spot

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A tender or sentimental feeling.
• noun A weak or vulnerable point.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun idiomatic A sentimental fondness or affection.
• noun A point of vulnerability in a defence
• noun A fontanelle

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun any membranous gap between the bones of the cranium in an infant or fetus
• noun a sentimental affection
• noun a place of especial vulnerability