Generally, this is a difficult question to answer. We all know a good logo design we we come across one, but it can be hard to define exactly what makes it a good logo. Generally, I tend to look at colour, concept and execution as the three factors that make up good logo design. For me, I think all three (or at least the last two) need to be of a high quality for the logo to be considered good; a good concept with poor execution will result in a bad logo, similarly, good execution of a poor concept will generally end up in a poor logo design.
Many logos you come across will have been driven by concept; so a real estate company logo will usually start with a house or building somewhere in the logo, and will be built from there. For corporate logo designs, it’s generally argued that the logo should be graphically simple and iconic; containing all company elements in one icon (name and what the company does). Typography (i.e. your font choices) should be subtle and should not detract from the overall concept (so they shouldn’t be distracting). Any icons used in the logo should be capable of standing without the text, without losing any meaning.
Execution generally refers to how well a logo matches concept, iconography and typography, as well as how well rendered the final version is. So poor execution would usually consist of a lack of any solid concept (just meaningless shapes or lines) coupled with poor rendering. If you’re including a low resolution image in your logo, it’s likely that the entire logo will be tainted by the low resolution and will end up being considered as a poor piece of execution. Even if you have a solid concept, it’s likely that it will be let-down by use of low res images and poor rendering. Logos like this one look especially poor when utilised within a web design, as the low res rendering will be made more apparant, particularly if grouped with other high resolution graphics.
Have a look yourself at some logos online and try and assess the elements mentioned above; chances are if you find a good logo that it will contain at a good concept and a high level of execution.