I have seen some terrain bugs in every private server i have played on.
But the most common thing i've seen in everyone is that players movement looks wrong. It is like when you turn right or left the player seems like doing spikes. For everyone that has played on Retail knows what i am talking about, its like movement goes with spikes.
The only fluent server i have played on like retail is Arena tournament. Is the only private server that has provided a really retail experience, and when you play on it you can prove this.
I don't know if developers know about this issue, but its really important. When you play on Feenix or another stupid private server you can see that Warrior charge movement look like it was insta-teleport, same for Death Grip from dk's. Also players in feenix look like they are teleporting when they walk (like it was Counter strke with 200ms, i don't know if this issue is because of the hosts).
Anyway, if you want make a server with a really retail experience, you should see on these stupid bugs. Meavy you thing this is pointless, but IS NOT. These issues makes you feel like if you were playing in a crap server. (and so it would be)
PD: Pls, dont make me record these movements that i am talking about.