You can be sued for anything in the U.S.
However, the US Supreme Court settled this issue in 1996 in the case of Lotus Dev. Corp. v. Borland Int'l, Inc. Borland copied the complete Lotus 1-2-3 menu structure in their Quatro Pro software product. The Supreme Court ruled in Borland's favor.
Borland vs Lotus So unless you are infringing on a patent, trademark, or design patent- you can copy the look and feel of another program.
Obviously people can sue for about anything, determining if they will prevail is another matter.
As to "look and feel", if it is part of the key branding of the product or service and has been registered with the Patent and Trademark office, then they will most likely win.
Consider McDonald's golden arches, clearly a part of the look and feel of a McDonalds hamburger stand, if you open a hamburger stand with your name and two yellow arches, you can virtually be guaranteed that McDonalds will win against you.
With software you will find that in addition to trademark elements there are numerous patents that have been filed with respect to interface design which may or may not be valid, but which you could run afoul of.
Since you are asking this in OnStartups, I am guessing that you have an idea for a product or a product in the works which you are concerned may put your company at risk in this area. Given that context, there are a few things you should consider:
IANAL and your mileage may vary :)