I've been thinking a lot about this issue lately. While I don't care at all for the time pressure and even reflex based gameplay of real time; I think there is a real trade off from doing quantised turns, and doing simultaneous movements.
The thing about quantised turns, that is, treating time for a single unit, or possibly several allied units, as separate from the rest (as in X-Com or Jagged Alliance) allows you to consider all the possible actions you could take in a more sane and isolated environment. This can make the game fun in a way that simultaneous movements wouldn't allow; perhaps we could even call it more "chess-like". In fact, bigger turns (that is, games that allow you to do more in a turn, usually through some kind of action points) might excel in this.
Simultaneous action games, games where every unit moves and acts at the same time, tend to have a different feel. Since there usually isn't a logic place to pause movement, the gameplay tends to be less optimised. You don't win by making sure you always end your turn behind cover or by saving aus to shoot the enemy if he walks in your field of vision. On the other hand, simultaneous movement isn't abstracted like turn based movement is. Some tactical options that are impossible in turn based combat become available with simultaneous movement. Some of these can be considered in turn based games, such as "overwatch" or whatever other names you might give to a unit attacking outside it's turn. But perhaps the worst problem is the issue with response. In a turn based game, units may take a lot longer to respond to what the enemy does than it is reasonable to expect, exactly because the turns are long.
I think there might be ways of further improving turn based combat so that it is more realistic while keeping the kind of gameplay they excel in. A possibility could be to have small turns for movement but larger turns for actions. So, for instance, you might move your character this or that way in the short turn, without much of a penalty, but giving up shooting an enemy for another might cost precious time units while your character drops his previous aim and focuses on the new target.