Generally, this is a difficult question to answer. We all know a good logo design when we're present with one, but it's not easy to clearly define exactly what makes it a strong logo. I tend to look at colour, concept and execution (with the last two being the most critical) as being the factors that contribute to good logo designs.

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 generally start with some variation on a house or building for example. For corporate logo design, it's pretty well accepted that the logo should be iconic, contain all company elements (name and area of industry) and be graphically simple. Typography (i.e. your font choices) should be subtle and should not detract from the overall concept (so they shouldn't be distracting). The icons should be capable of standing on their own, without the text.

Execution normally refers to how well you can match typography, iconography and concept, and also how well rendered the overall logo 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 using a low resolution image as the base of your logo, it's likely that the execution will be poor, as the logo will inevitably be low resolution. 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 resolution rendering will be made even more apparent.

Have a look online at some brand logos and see if you can assess the concept and execution; chances are if it's a good logo at least one of those factors will be original and unique, and the best logos you find should have achieved a high degree of both concept and execution.