There's no solid definition for what a logo is or should be. You won't find too many designers that agree 100% on what makes a good one or a bad one. Taste is subjective too, even really terrible logos have their fans. But, I can tell you what I think a good logo is.
I'm not going to tell you any magical secrets to logo design or that every logo should follow the same set of rules. Some logos need to do different things than others and they follow their own rules. Some make use of a clever idea and pull something personal from the thing it represents. Others are just for "performance" and need to do their duty, like sit in the corner of a webpage and identify what that page is. Some represent a culture with a specific concept, pattern or color (sports fans think of the Tampa Bay Buccaneers and Seattle Mariners), while others express a single idea well and are built to be absorbed by it's audience and become a permanent mark of the culture later (think Indianapolis Colts and Chicago Cubs).
Either way is fine, but understand that no logo is born with emotional value. That is earned over time, which we call equity. Most people don't like change and would rather their favorite brand hang on to the shitty logo they've always known than replace it. If it's a new logo/brand there will be many haters of it once it's debuted. It gets understood and accepted and loved over time.
What I will tell you about logo design is only what I can tell you. My views on logos are defined by learning from other designers, my own education, and my own experience. . . and a lot of time being a dork and thinking about logos. If I could sum up what a logo is, it is a thumbprint. To sum up what it does, it identifies something, then communicates a message about it.
Logos, like all design, send a message; and whether you're doing something personal and clever or just something to perform well, there is always a conversation with it's audience. Design is not all about aesthetics. Aesthetics are the least important thing about design. Your ideas and connotations and emotional responses to design are what makes it valuable and good. Form follows emotion. And study your history too. Colors and symbols can easily pull mental images of the wrong thing if you're too focused on aesthetics and visual performance. I like to use the example of the worst idea in Pro sports branding history: Putting a team named the New England Patriots in a red jersey and white pants.
That football uniform looks great, doesn't it? . . . But the concept fails.
What I use to define a good logo is a set of measures that I believe in fully, but I don't think all of them need to apply to every logo. Again, some logos just need to do different things. I start with the idea and emotional expressions. You don't need all 3 every time, but if you can capture the 1.) Personality 2.) Culture 3.) History, of something in a single logo, you've got a great concept. And those are listed in order of importance. Design always has a personality though, so make sure you're expressing the right one.
The next set of measures is about the performance of the logo and the more tangible aspects of it. These are the duties that need to be carried out by the design. If a logo "works" well, then it does these things well. Sometimes, a few of these are all you need. There's some overlap too, for instance simplicity helps visibility and memorability, good color choices help with distinctiveness, etc.
Visibility – Readability and clear form, even at small sizes.
Application – How and where it's used. If it's going to live primarily on a football helmet, then that's the main thing you have to focus on. Design for that thing first. Also think of environment. If everything that surrounds it is green, then green probably isn't the best color to go with.
Distinctiveness – Different from brand's competitors. It has to be able to be different enough to be easily noticed.
Color – The color has to be pleasing as well as having qualities on this list of it's own like visibility and distinctiveness. A great logo can be ruined by a bad palette. A single color can express the brand's full personality. I believe color is one of the most important elements of design and you can never know too much about color theory and meaning. BE GOOD AT CHOOSING COLOR!
Simplicity – Helps memorability, timelessness, and reproduction across multiple mediums and sizes. A logo dosn't have to work in black and white anymore, that's dated thinking. But it is still good practice for seeing simplicity in form.
Memorability - You want people to be able to recall your logo without being reminded of it. They should be able to clearly describe the idea. "The football team with the G on the helmet" is better than "The football team with the circle and weird brown thing in the middle"
Timelessness – Dosen't use design trends or rely on new technology. Sometimes It's okay to follow those trends. Sometimes It's okay to build a logo with a short shelf life. But not if you're going for timelessness. It should never show it's age.
Equity - The longer a logo is around, the more it soaks up the brand of a company and becomes a symbol of what the audience thinks of the product. Coca-Cola's logo isn't great typography, but the sight of the logo usually brings thoughts of happiness and togetherness, because thats what the brand is, and that's the emotional value. If a logo has good equity, don't change it.
Modularity – Versatility. Can it be a mascot? Some logos have to be.
Appropriateness - It should fit the brand goals of the company, and the culture of it's audience. I've always felt the Buffalo Sabers "Buffaslug" was a great logo, just for a football team instead of a hockey team. Still, I'm one to push the appropriateness level to the very edge, doing what's right for the brand and letting the logo find it's audience. That's kind of the Apple way of doing things.
Descriptiveness - It needs to communicate what you do. This is more for lesser known brands. You probably don't want to use a snare drum in the logo for a plumber.
The end result of a good logo should be something people are proud of. Something that you wouldn't mind wearing on a t-shirt. And, if it's a sports team especially, try not to design something that it's rivals can easily make fun of. My personal redesign of the Indianapolis Colt's horseshoe was said multiple times to look like a toilet seat. . . well, fuck!
Check out some of my favorite logo designers:
Fraser Davidson - Marc Verlander - Kris Bazen - David Airey - Tin Bacic - Jan Zabransky - Gert Van Duinen -