A druid has a higher base attack, so he is going to be more likely to conferm that critical. he is also more likely to hit a stardard ac in a high dex wildshape rather then a wizard or sorc. A wizard or sorc could metamagic the hell out of this spell... but they are going to have a really sucky time trying to hit a creatures full ac. even if they do crit they are likely not to conferm the cirt, and then its only as good as a sorcheing ray.
A druid could take improved crit at level 12. Thats when most druid take dragon wildshape. So a druid could choose to take improved crit, but... more likely they are going to take dragon wildshape. Now it would make a good level 15 or 18 feat but the spell is still only level 2 and thus negated by globe of invulnrability. so using a level 15 feat to have it negated by a level 4 spell is not the best thing to do in my opinion.
a Wizard/Sorcerer has to use thier level 18 feat to take improved critical. and again... same problem... they have to hit a actual ac.
Im not saying this spell isent awsome. I am just saying its balance is more staggard. Under most sercumstances, its weaker then scortching ray, in others its alot more power then scortching ray.