After deleting all nodes, how do I reset the node ID to start from 1 again?
ALTER TABLE `node` AUTO_INCREMENT = 1;


P.S - Others who have replied on this question: I just did a small check after deleting content and the associated content is indeed deleted, node_revisions and field_* tables are empty. node_delete says the same - http://api.drupal.org/api/drupal/modules--node--node.module/function/node_delete_multiple/7

Sometimes on migrations projects when you migrate from other platforms (ex. ASP + MSSQL), sometimes it becomes unavoidable to reset the nid due to lot of factors. I am sure it could be avoided but time/code tradeoff is always there.