Deleting Content Manually
If you delete a row directly from the node table you can expect:


At best - you will just have a bunch of orphaned data in the system. Tables containing this orphaned data include, but are by no means limited to,


node_revision
taxonomy_index
url_alias
search_node_links
Any number of field_data_FIELD_NAME and field_revision_FIELD_NAME tables


Those are just a few of the core tables that can be affected; there are others, and many contrib modules will install tables to store node-related data. Those will also potentially contain orphaned data.
At worst - there's no telling how this could affect your site, it completely depends on how it's configured. You might get lucky and experience nothing but the orphaned data, but I wouldn't advise counting on that. Before you do anything, take a backup of your data.