Yeah, there are a few problems that really prevent ganks. A hand-full actually...
1) Stacking move speed = can't die.
2) Line of sight is very long, and on most maps, the combination of many towers and constant creep waves gives you line of sight over everything except their base and the unguarded flags.
3) There are no obstacles other than untraversible pitfalls that provide a buffer for line of sight.
4) Most maps don't provide alternate routes to main routes that aren't watched by towers/creep waves.
They really need to work on the maps so that they aren't completely wide open brick roads for ADD kids to run straight across.