I am very excited about the new CRM 2013 release. There are a lot of new functionality coming out with the new release.
There are couple of obvious ones like better look and feed and flatter UI as mentioned in the posts below:
From technical point of view there are few features that will help performance for sure.
There is no more extension tables. Till CRM 2011 each entity had extension table and data was retrieved using filtered view which would fetch data from 2 base tables. With CRM 2013, extension tables are merged with the base table. Hence there is only one table / entity. This is a huge gain from performance point of view for sure.
CRM 2013 loving it.