Why won't my leads sync to SFDC without a company name?

It's a salesforce requirement to have a company name.  Check out this article and note the callout on the right hand side:  http://community.marketo.com/MarketoArticle?id=kA050000000LAAhCAO

You should check your SFDC and try to create a new lead. You'll see that several fields have the required red marker. The default, as Cheryl notes is for Last, Company, and Email Address. Occasionally, a company will modify this.

Either way, Marketo cannot create a new record if the required fields are not filled in.

Go to admin and go to the SF sync options. Populate the default Company name field with "-" or "n/a".