Update Rollup 10 for CRM 2011 introduced changes to the way quick find behaves.
Quick Find Record limits
A new option in System Settings, Enable Quick Find record limits prevents searches that return 10,000 or more records. Searches that exceed the limit receive a warning instead of the search results.
The way Quick Find works is changed in UR10. The new method can be disabled by setting the OrgDbOrgSetting, EnableQuickFindOptimization to 0.
CRM 2013 - Indexes for Quick Find columns
In CRM 2013, indexes are created for Quick Find columns automatically (when a column is added as a search column). The QuickFindEntityIndexLimit deployment setting controls the maximum number of search columns for which an index is created, the default is twenty.
Some blogs claim that indexes are not created for columns that have more than 900 characters. Need to find an official source for this claim.