We started a MP game about a month ago.. on turn 27 now. The same peeps have been in the hall of fame since turn 2. Most of them are dead, I know other commanders have had more experience than some of the peeps in HoF.
There are two factors to entering the hall of fame. One, as noted, is experience. The other is kill count. The latter appears to count more, but I don't know the exact ratio. I'd guess the famous martyrs had a number of kills to their name.