What to look for in an induction stovetop?

1. If you like to cook using 3 or more elements at once, ensure you buy a sufficiently powered model. There's nothing more frustrating than upping the power on one element and watching another one reduce at the same time.
2. Touch controls look nice but they're horrible from a usability standpoint. I always had problems with responsiveness with wet or greasy fingers.
3. Cockroaches love induction. I don't know if the frequency attracts them or it's just the heat but they will come and eat out the wiring, even if you've never spotted one in the house before. It's a common problem as it's hard to seal the electronics for thermal reasons. Board replacements are expensive. It's one item I would consider an extended warranty on.
4. As for any 2" cooktop, larger pot/pan sizes can't be accommodated if you're using 3 or more elements. Go 2'8" minimum.

It is always best to try the product before buying it. As for the touch controls I prefer a control panel that is angled, not flat. This way I can accommodate larger cookware without it touching the controls. If you are using a multi burner cooktop ask if all the burners can be operated at full power together. Most induction cooktops use power varience technology and they share the total power between the burners. Definitely go through reviews before buying.

To answer your question, the advice I would give someone on buying a stovetop is the same as the advice I'd give someone for buying anything else: do as much research as possible before buying. Don't let a salesman tell you what you need; find out what's available and what you want/need, and then decide based on how much you're willing to spend. If you're not an expert on stovetops (which I should point out that I am not) there are lots of buying guides out there that outline the features of various technologies (try not to get all your information from one place, especially not if it's a company trying to advertise their products). Various authors writing these "consumer buying guides" will often point out their own suggestions that you might find helpful. Here are a couple links for your reading:

US Appliance

Consumer Reports

Try to look around and always have as much information as you can get before making a decision. It's a bit of work but it always pays off.