When it comes to city defense...the current system is totally unbalanced in favor of the defender. Players can chose to take advantage of this by filling a city to the brim with troops..every unit gaining the city bonus and every command/stacking bonus of every other unit in the city. the worst is a hero sitting in a city with a horde of units...if you attack with your own horde you will likely boost this hero to an unmanageable level.
Usually, when I see an opponent do this, I chuckle and just march my troops around to attack a softer target....but I have faced some situations where this was not a very attractive option. A heavily defended city within striking range of several of my cities is a nightmare situation. It slows the game down by forcing me to focus on defense (unless I am willing to give up the cities).
Being able to self raze would be helpful in this situation..but, still seems to be rewarding this turtle strategy.
It would be nice if there was some reason for players to moderate the number of units in a city. I recently proposed a 'plague scroll' for just this purpose. Or maybe just a chance of plague when unit numbers get too high?
KGB and I discussed a siege mechanic a while ago...but now with more games under my belt, I think that a siege mechanic should starve units..or start a countdown to automatic city conquest.
The basic idea is to discourage turtling and static play, thereby encouraging active, enjoyable games.