The amount of entertainment you can gain from a freebuild depends on:
Lag: If either your connection or the host's server connection is bad, then building can be more difficult
Players: You can get really great servers with helpful people, or just spam servers with idiots building 1x1 blocks everywhere
Dedicated: It won't shut down, or is less likely to. However, you run the risk of lack of a rule-enforcing admin
Admin: Good admins = No rule breaking = Good server
Capacity: Larger servers can be more fun, yet have more buildings in place and blocking areas
Map: Goes with capacity. If a map has more building space (eg. Slate) it is generally easier to find a nice spot to build than, say, the Bedroom.
Minigames and their location: Minigames are great in a freebuild; You can stop building for a while and blow people to hell. However, minigames amongst non-joined players who want to build can be annoying.
There are more, but I guess it depends on user preference more than anything. I personally avoid them because I can build far more easily in single player and don't get distracted doing so.