The biggest problem I can see is that of stacking effect. Often a more generic term is actually the best (Singe alone!) so pairing it with another term would make it simultaneously less effective and more expensive- unless rules were put in place.

Basically, since the system is based on words, you are missing a grammar. In a system such as yours, you should consider a baseline grammar, preferably which each component adding more power.

VERY BASIC EXAMPLE:

Your spell casting language could start with the structure: VERB OBJECT

VERB is the action to be performed, OBJECT is the object it is performed upon. All spells would need to include those components, although I suppose if the object was removed, it could always refer to the caster.

Next you would create a number of generic VERBs- fire, ice, positive energy, negative energy, air, electricity, growth, shrink, strength, weakness, summon... etc.

ALTERNATE: a more basic system might include only basic verbs: Harm, Heal, Augment, Supress, Create, etc.... This would make modifiers (not intoduced yet) more important.

And then you would create some generic OBJECTs(for D&D, how about starting with creature type?)- humanoids, giants, animals, magical beasts, abberations, etc. Water, Area, etc could also be established.

Finally you would understand exactly how these terms interact. A good rule of thumb might be 1d6 of VERB effect to OBJECT.

Regarding objects, I personally think you should have specific OBJECTs be more potent, so knowing the generic term for Wolf or Human would make the spell more effective, whereas knowing the specific name of a specific dragon would make the VERB that much more effective against that Object.

At low levels, a PC could start with a limited number of OBJECT's and a limited number of VERB's, and by combining the two he would end up with the end effects.

The final touch are the modifiers. They would handle quantity, size, and power level (how many die of effect are applied.) I will think more about this before posting too much more.

Note, if the VERBs are descriptive: Fire, Ice, Positive Energy, etc, the verbs hold the majority of the power in the system. More basic VERBs (augment, heal, harm) shift some of this power to the modifiers.