Changes 53.1:
Data from the CLDR 25 release: Many bug fixes
Time zone data: 2014b,  including post CLDR 25 time zone data update to CLDR.
U+20BD Ruble Sign added (from Unicode 7.0, otherwise ICU 53 still uses Unicode 6.3)
MeasureFormat API for new units in CLDR 24
Hoisted setContext/getContext from SimpleDateFormat to DateFormat, implement context-sensitive capitalization of relative dates
Added setContext/getContext methods to NumberFormat (and unum_setContext/unum_getContext for UNumberFormat), implement context-sensitive number formatting (for RBNF spellout)
Improved lenient date parsing consistency between ICU4C and ICU4J, add finer-grained control of date parsing leniency
Fixed numeric rounding in TimeUnitFormat
Fixes to Unicode 6.3 bidirectional algorithm implementations to behave exactly like reference implementations
Improved UTF-16 charset detection
Collation code re-implemented
Many bugs fixed, some enhancements implemented (link for ticket query)
Passes full UCA conformance tests now
Updated to UCA 6.3/CLDR 24 root collation
Performance: C++ UTF-8 and Java string comparisons significantly faster (very small reduction for C++ UTF-16)
Collation data size (uncompressed) reduced from 4.48MB (ICU 52) to 2.62MB
New data format, removed empty files, fixed genrb bug
More APIs function when collation rule strings have been omitted from the data files (e.g., getTailoredSet())
Java Collator.compare(Object, Object) now works with CharSequence, not just String
Java Collator base class (does not apply to RuleBasedCollator instances): getters for strength, decomposition mode, and locales return hardcoded default values; their setters do nothing
Rule syntax and semantics tightened and improved, matching LDML 25 Collation Rule Syntax
In particular, rule chains now must start with a reset.
Setting of variableTop deprecated, and not supported in rule syntax any more
Replaced by the new maxVariable setting; see LDML 25 Collation Settings
Accounting format supported in NumberFormat
RelativeDateTimeFormatter class for formatting relative times such as "3 weeks ago" or "next Tuesday."
Updated Spoof Checker for Unicode Security Standard version 6.3.

(adam)