A good abstract is a clear summary of the article. A bad abstract is not clear, overly terse (rare) or overly wordy, or even misleading. 

Nature gives detailed guidelines for how to write a well crafted abstract. See their guide for an annotated abstract (https://www.nature.com/nature/for-authors/formatting-guide). 
    - "One or two sentences providing a **basic introduction** to the field, comprehensible to a scientist in any discipline."
    - "Two to three sentences of **more detailed background**, comprehensible to scientists in related disciplines."
    - "One sentence clearly stating the **general problem** being addressed by this particular study."
    - "One sentence summarizing the main result (with the words “**here we
show**” or their equivalent)."
    - "Two or three sentences explaining what the **main result** reveals in direct comparison to what was thought to be the case previously, or how the main result adds to previous knowledge."
    - "One or two sentences to put the results into a more **general context**."
    - "Two or three sentences to provide a **broader perspective**, readily comprehensible to a scientist in any discipline [...]."
In essence you start with a broad introduction, then a more focused introduction, focus in on your question and main result, then move back to a broad perspective of the implications of the result, and then an even broader perspective. 

A misleading example is "The health effects of a Roundup-tolerant genetically modified maize [...] were studied 2 years in rats. In females, all treated groups died 2–3 times more than controls, and more rapidly. This difference was visible in 3 male groups fed GMOs. [...] Females developed large mammary tumors almost always more often than and before controls, [...] Males presented 4 times more large palpable tumors than controls which occurred up to 600 days earlier. Biochemistry data confirmed very significant kidney chronic deficiencies; for all treatments and both sexes, 76% of the altered parameters were kidney related. These results can be explained by [...] by the overexpression of the transgene in the GMO and its metabolic consequences." [[seralini_et_al_2012|Séralini et al. 2012]]

The results presented in the paper did not support these statements in the abstract.