Dead end? Dead map.
Every room or hallway should have at least two entrances. You may ignore this rule for very small rooms which are basically impossible to defend. However, even in those circumstances, I would recommend that you try to avoid dead-ends whenever possible.
All roads lead to Rome
The gist of this rule is that every exit from your team's base should inevitably lead to the opposing team's base. ATCS and Tremor are good examples (although they are relatively small maps). Karith is a good example also; even though it's not completely linear, you really can't get turned around and end up walking in circles.
There are plenty of decent maps which do not follow this rule. Procyon is an excellent example, and Niveus to a lesser extent. However, maps which do not follow this rule are much more difficult to learn, leading to major player frustration for several games until the layout is memorized. If your map does not excel in almost every other area, this style of layout (let's call it extremely non-linear) will cause players to give up on the map before it becomes enjoyable.
Teleporters are bad
Do not put teleporters in your map. They cause all sorts of gameplay problems, not the least of which is telefragging. Players get lost more easily, etc... Teleporters should be reserved for single-player games and novelty atrium deathmatch maps.
Elevators are bad
Elevators get camped. Tyrants get squished. Teamwork becomes nigh-impossible since teammates can't jump onto the elevator at the exact same time. In short... just avoid elevators completely.
