I am going to say that I like flexible magical systems, preferably skill based. But Freeform magic actually sucks.
Freeform magic is normally defined as "any reasonable magikal effect" that works dramatically with the story. Freeform magic is normally only used in freeform roleplaying games. You know what I am talking about. Games without rules, but where players post up parts of the communal "story" on the forum or email.
So if they need a light area spell, it goes off. If they need a "smite evil things within the immediate area" spell, it goes off. You begin to see...
The main requisite of a freeform game system is communal trust, i.e. everyone is working together to make a story, rather than competeing against the world/GM to achieve X. (That is why you are not allowed to do anything against anyone without the creating character's permission, including against bad guys not of your own creation). Most table top games are not based on communal trust. While the group might work together to built plots and develop characters, the game portion of the rules (randomness and limits on what you can do) create a competive element to the game.
The closest things to a freeform spell system in conventional gaming are the magic systems for Mage the Ascention and Ars Magica.
Mage's system defines levels of effects the character can achieve. So if you have life-2, you can generate any effect that either senses life (Life-1) or "adjust" an existing biological process (life2). Each tier is given numerous examples and shows how you can combine them all together.
Ars Magica is a "skill" driven system. You "knowledge" of a given element and magical verb, defines your chance of success performing a given skill. If the spell one that you have learned (or created and learned), your score in the element effected and the verb associated determines your chance to cast. You could still cast spells that you had not learned, but they were performed at a minus.
Mage had "well defined" levels of effect for each tier. But even these well defined areas did not balance the system. In the original mage (ed1), a begining mage could create an atomic explosion by creating unstable matter (showing that they people that playtested the game did not understand science). That was cleaned up in second edition, and more example rotes were given so people could actually understand what was possible with what tier of effect.
Ars Magica went the other way, with a huge list of example spells. Players were still creating their own spells, but they could use the "common" spell if they wanted. This has the advantage as this huge backlog of allowed GMs to have a variety of spells to NPCs, rather than spend time and effort to create unique spells for each mage. The backlong served as examples so GM's could determine skill levels and power effects appropriate for what the players wanted.
What CP had defined is a "non standard" spell list. Instead of 22 possible spells per level (as per DnD and most systems that use a D20ish spell system). He is going to allow the players to create their own spell effects. How many spells are castable on a given day is defined by the game system he is using (which is a homebrew if I remember correctly).
Every spell system needs guidelines and limits. Guidelines define what is appropriate for the setting, in terms of effects (what it can do), power (how much of what it does), and feel (in a gothic world, having a happiness spell just is not appropriate, while in a Slayer's game having a summon elder god spell is not appropriate). Limits restrict what is and is not possible. These limits can radically change the feel of the game. Take away resurrection and easy magical healing, and players will need to adopt a very different style of play (or roll up lots of characters) than when these effects are plentiful.
Now even Mage and Ars have these guideleins and limits. Any magic system that is not destined for GM hatred has them. For everyone's safety define the spell by end effects, not about what it can do.
So CP, what are you going to use to show your players guidelines and limits for the game? Or are you just going to let them throw things at you until you either relent or they figure out what your unwritten rules are.
I mean, can I create a small electrical arc spell that occurs between my hands. Does very little actual damage to what is between them. Seems innocent enough. Until the first time the mage does a grapple attack and sets the spell off... basically stopping the target's heart. Oops. A death save spell has been created.
So simple is not neccisarily better. Simple allows more possible ways for the players to "whack the system" and fewer rules that allow the GM to disallow certain spells (besides using the GM fiat... which can seem heavy handed). Some magic systems are not intended to be abused, but are because the players are comming from a different view of the world (mage's atomic issue).
While I personally hate spell slot systems (D20 base magic), well defined, well playtested rules, are always the way to go.