> RPi has definitely enough horsepower to run a small domain
Maybe enough for the DC part (i.e. Kerberos server, LDAP server, ntpd) but not enough for a fileserver given the Pi is still 100 MBit and limited by its USB ethernet connection, as well as that it doesn't have eSATA, mSSD or any other high-performance storage option. Plus SMB/CIFS implementations are known for performance issues.
Also, a Pi is nowhere near reliable enough for running something as mission critical as an AD server. Good luck when your micro-SD card gets corrupted, e.g. due to power fluctuations. And you WILL get corruptions, especially if you have high write throughput.
Maybe enough for the DC part (i.e. Kerberos server, LDAP server, ntpd) but not enough for a fileserver given the Pi is still 100 MBit and limited by its USB ethernet connection, as well as that it doesn't have eSATA, mSSD or any other high-performance storage option. Plus SMB/CIFS implementations are known for performance issues.
Also, a Pi is nowhere near reliable enough for running something as mission critical as an AD server. Good luck when your micro-SD card gets corrupted, e.g. due to power fluctuations. And you WILL get corruptions, especially if you have high write throughput.