This doesn't go very indepth at all and in fact some of these points are useless or counter productive.
When I sign up to a forum having a welcome message deters me. In fact I will often close it. When I put a welcome message on one of my forums it didn't make a lick of difference and while some people complained I still had people email me asking how to post even though the welcome email had an image tutorial and linked to the video version. I can't be more descriptive than a video. In addition to that people who had a welcome message they were ignoring often missed personal messages and other announcements because they'd [mentally] blocked out the new message alert.
Also regarding helpfulness that can help a little but not always. If you're in a support niche people will get their answers and vamoose. The best way to retain members is to relate to them and show them understanding and praise - you have to build a bond with them. No one is coming back for the forum they're coming back for the community.
And regarding activity you definitely need activity....if there is no reply to their topic for a week they likely wont hang around no matter how helpful or nice you are.