Wizards of ALPHA-30

JB is right, magic in D&D can become pretty mundane.

There are plenty of good suggestions in the comments to JB's post.  I thought rather than a good suggestion I would throw out a wild idea! 

Magic-Users can cast as many spells as they want in a day, but if one fails they're done for the day. 
It takes a full round of focusing and gesticulation for the M-U to generate a spell effect.  during this round the player writes a word that starts with one of the letters he is allowed on that day.  The DM writes a number that the PC must roll over on a D30 to successfully cast that spell.

Magic-Users can't use the same spell twice in the same day.

The Magic-User's level is the number of times they get to roll a D30 on this chart each day:

1 : A
2 : B
3 : C
4 : D
5 : E
6 : F
7 : G
8 : H
9 : I
10 : J
11 : K
12 : L
13 : M
14 : N
15 : O
16 : P
17 : Q
18 : R
19 : S
20 : T
21 : U
22 : V
23 : X
24 : Y
25 : Z
26 : Same leter as the previous day.
27 : Roll again.  The M-U will have to roll for a new letter each time they cast a spell today.
28 : Roll again.  The M-U will be able to use this letter for D6 days.
29 : Roll again twice.
30 : Choose any letter


The M-U character meditating for 30 minutes in game can get the Player 30 seconds with a dictionary.
 
There is a lot riding on the DM's judgement with the casting roll.  By writting down "30" the DM can make a spell automatically fail.  If I were going to try to DM this I would make a lot of spell effects really hard or impossible, and then I'd let the PC research and find material spell components, rituals, symbols and the like that would make a spell more likely to succeed.  I think I would also let them "research" certain words to get desired spell effects, and to get more powerful spell effects I might make certain words have side effects.

As for the words themselves, really creative players could get away with a lot.  It is Magic!  Maybe the word is a simple description of what the caster intends for the target, "sleep" or "stumble".  But then maybe the word is the name of a demon, god or otherworldly creature that the caster summons or communes with by saying it's name.