The Wizards team building DnD Next regularly puts out blog posts about their design ideas along with their actual playtest documents. They have a lot of interesting thoughts, and MAN am I jealous of their art department (since my art department consists of just one dude, and he isn't any good at drawing). Their latest post talks about spell design, in particular the save or die (SoD) mechanic. In the old days Fireball was absurd but in 3rd edition people focused more on SoD attacks because of monster HP inflation. That lead to some weird stuff, where beating monsters down was useless because at some point they would fail a save and just die regardless of their HP total. The article talks about changing SoD spells into spells that slowly kill the enemy over several rounds and with several rolls; eventually turning the monster to stone or somesuch.
The trouble is that when wizards have effective ways to kill monsters that entirely bypass HP and everyone else is forced to just do damage the party is going to end up with bizarre situations. The wizard can start casting a '3 rounds and you die' spell and according to the article the party will then spend their time trying to slow down or hamper the enemy from coming to kill the wizard. Unfortunately this is going to be a complete mess because if the party isn't trying to hurt the monster, just slow it down, and the monster makes its saving throw, then the party is no further ahead after three full rounds. If instead the party just beats the monster down while the spell does its work then the spell is probably terrible - using a SoD spell against an opponent who is already dead or nearly there is wasteful and pointless. Simply put, if the party doesn't need to SoD a monster because they can beat it down then SoD is awful. If they can't beat it down and they need to SoD it to win, then the party dies if the monster makes its roll. These are poor options.
SoD spells are going to work best when they directly interact with HP. For example, a SoD spell that only works when the enemy is bloodied (below 50% HP) would be fine. It won't let you beat something that is totally out of your league, nor end a fight on round 1. It will, however, let you do nasty things to enemies that are already beat down, which is fine because it interacts nicely with the rest of the party. "Get that dragon's attention and wear him down a bit so I can line up my KILLYOUFOO spell!" Of course you need a mechanic like bloodied from 4th edition DnD for this so players know when opponents will be vulnerable to a SoD attack. This makes the wizard a team player instead of a boss killing machine.