I'll try to tell you what they mean - point by point
Different subnets: WARNING: Not all of your nameservers are in different subnets
What this means is that your nameservers exist on the same subnet. If this network is to go down, people will not be able to resolve your DNS.
Different autonomous systems- WARNING: Single point of failure
Basically the same as the last one, except this means that your DNS servers use the same "AS". An "AS" is a routing term used in BGP. Basically, an AS is a "network" on the internet. If your nameserves are in the same AS, and the internet connection to that AS drops, your nameservers once again become unreachable.
SOA MNAME entry - WARNING: SOA MNAME (mydomain.com) is not listed as a primary nameserver at your parent nameserver!
This one you can actually fixquite easily. The SOA record looks like this:
;; ANSWER SECTION:
google.com. 86400 IN SOA ns1.google.com. dns-admin.google.com. 1399740 7200 1800 1209600 300
You have the domain name, TTL, protocol, record type. Then "mname" which should be the name of the PRIMARY domain name server, followed by "rname" which is the zone admin email (using a . instead of an @) and then serial #, refresh, retry, expire, and a default Time-To-Live for records.
Your domains mname should be the primary name server.