Last week, while trying to upgrade a client site from 1.4.x to 1.6.x , I faced a weird issue, where the index management was getting stuck at Tag Aggregation Data. Here I what I did yo fix it :
1. Checked for InnoDB database collation.2
2. Verified the database with Database Repair Tool.
3. Then removed all the 4 tables related to TAGS , and dropped it. Then exported it from a clean 1.6 database and imported it. Cleaned the cache and reindexed then.
4. There is also a fix here, which worked for many, http://www.magentocommerce.com/boards/viewthread/264050/
Article source: http://www.learnmagento.org/magento-tutorials/magento-index-error-on-tag-aggregation-data/