Even though it is a mere contrib and as such not included in the top of the release notes, I predict pg_stat_statements will be a huge leap in productivity in terms of tuning database indexes, which nobody gets around to getting time to do because it's a total pain to do the reporting about what is actually slow. Now it'll be pretty easy by comparison and I hope people will write even more reporting tools to make it trivial to do a reasonably good job.
An index suggestion tool may be more credibly in the cards of the moderately distant future in the future now that this critical work is done.
Is there any close equivalent for mysql, which we use at $work? The slow query log is the closest I know about, but doesn't seem nearly as useful (or detailed).