Bells and whistles refers to non-essential features, visual or functional, that are an enhancement to the object. What is the opposite of bells and whistles?
I would call those the nuts and bolts.
- Fig. the mundane workings of something; the basics of something. (See also get down to the nuts and bolts.)
I want you to learn how to write well. You have to know the nuts and bolts of writing. She's got a lot of good, general ideas, but when it comes to the nuts and bolts of getting something done, she's no good.
I would say that things that are opposites of bells and whistles (that is, they are not optional) are essential (adj.) or essentials (noun). Another noun that would work is necessity.
Edit: Since a more metaphorical example was asked for, I shall provide one that I think fits decently: meat and potatoes. The meat and potatoes are the basic, necessary things; this is the opposite of bells and whistles.
The company I work for uses the term "table stakes"
In business, table stakes also refers to the minimum entry requirement for a market or business arrangement. It can refer to pricing, cost models, technology, or other capability that represents a minimum requirement to have a credible competitive starting position in a market or other business arrangement. For example, if you want to be a Wireless service provider the table stakes are the basic features you need to have in order to be in that business to achieve foundation capability - Network, Handsets, a data service, a mail server, etc. Beyond that, real competitive advantage comes from additional nimbleness and cost or product differentiation.
I guess I am the lone voice of dissent, but I would argue that meat and potatoes and bare bones have a slightly different intent and connotation. Bare bones I think is a perfect antonym to bells and whistles, while meat and potatoes, not so much.
Bells and whistles - Describes non-essential add-ons but does NOT seem pejorative in meaning. I'd say a car with SatNav, leather seats and satellite radio had all the bells and whistles but I would certainly not mean that those extras were frivolous. All the bells and whistles sounds generally positive to me.
Bare bones sounds just slightly pejorative to me. Bare bones comes off to me as if you would take more bells and whistles if they were an option. If I wanted a bare bones car it would be just transportation, without the extra because I couldn't or wouldn't pay for them.
As opposed to Meat and Potatoes, which while close, sounds a little more positive. Meat and potatoes sounds to me like I want to keep it simple, and view sticking to the functional heart of something as preferable.
I buy a Mustang with all the bells and whistles because I can afford to splurge.
I buy a bare bones Mustang because I can't afford more, or intend to do all the upgrades myself.
I buy a meat and potatoes Mustang because I like a straight up simple functional performance car and don't want or need all those frivolous add-ons.
Just my personal feeling when I hear those terms. On a side note, where I am from, I think the most common direct antonym to bells and whistles is "no-frills".