You'll need to put thousands (maybe even hundreds) of questions for some species.
You can put this with nice image/grapics of serrated/non-serrated leaves. This idea remind me of Akinator, where you can answer Yes, Proabbly, Don'tKnow, ProbablyNot, No. (http://en.akinator.com/ ) (You also have to consider the possibility of a mistake in one of the answers.)
One problem is that plants have different names in different parts of the word, or even in different parts of the same country. Another problem is that the same name may describe different plants in different places. (Perhaps the Wikipedia article can be the "official" identifier.)
I can probably identify thousands of genus plants at least at first sight but is not easy and sometimes is impossible. There are about 3000 spurge species for example (and maybe ten real specialists in the genus Euphorbia in the entire world).
There are also some legal troubles in the field to care for. Some typical recurrent doubts in plant identification in my experience are:
1-"How can I cultivate peyote/cannabis/some ilegal drug in my house?, Is this a male or female plant? Is this cactus in my public park a San Pedro?"
2-"Is this leaf/mushrom/berry that I'm cooking edible or poisonous? (typically with a blurry small photo of some green poor thing that falls between a child in cucumber disguise and an alligator)."
Both 1 and 2 situations could easily escalate to a lawsuit against you. Assure you to put a big disclaimer note in the terms of use.
3-"Urgent!, Urgent!, I want you to do my schoolwork. Identify my entire herbarium, Is for yesterday!."
This guy will not even remember to thank you later most of the times, nor will consider to paid a cent for this (I will gladly charge him for my time if I could otherwise).
Having botanists do this is good for accuracy, horrible for scalability. A neater solution would be to, as the botanists identify species, fill in a number of key datum for them, e.g.
Where is it growing?
What colour are the flowers?
How many nodes do the leaves have?
Are the leaves serrated?
Is this plant parasitic?
etc.
Ask enough appropriate questions and you can identify the vast majority of cases - essentially "animal, mineral or vegetable" - thus freeing up the botanists to do the really tricky ones.