Here are the things we limit for server stability.
Speed hacks are limited to a generous 36bps in the overworld and unlimited in the nether. Packet movement hacks are limited by a packet limiter.
The Nether Roof is disabled because fast travel on the roof can lead to a full disk. Additionally, disabling the nether roof promotes public works projects like the nether highways.
An automatic anti-spam is in place for new members with less than 24 hours of playtime. Additionally, a select few unnamed players have mute ability to keep chat clean and usable.