Is fluoride toxic, and how worried should I be about it?
First of all: Yes, fluoride is toxic, but the toxicity depends largely on the form (soluble vs. unsoluble, which fluoride salt etc.) occurs. It also depends on the environment since insoluble salts which are subjected to strong acids can release fluorine ions.
The certain toxic dose for adults is 32-64mg/kg body weight, a 75kg adult needs to take up between 2,4 and 4,8 grams of fluoride ions (for sodium fluoride this would be between 5,3 and 10,6 gr of the salt). For children this is about 5mg/kg body weight.
For the water the added dose of fluorine is much lower, somewhere between 0,7 and 1.2 mg/l. Other sources of fluroride are food (especially sea food) and also black tea.

The toxicity of fluoride is mediated by different ways: First it forms an insoluble salt with calcium in the cells, effectively causing hypocalcemia. Calcium is an important second messenger in different signal transduction pathways as well as highly important for the functioning of nerve cells (which also explains why it is neurotoxic in the development of nerve and brain cells). It also causes the inhibition of a number of different enzymes as enolase (disturbs the glycolysis) but also phosphatases and katalase. For all of this have a look into the Wikipedia if you want to know more details.

About the paper: It is a review of published research articles which sums up the situation in this field. The paper mentions fluoride only twice: Once on the abstract and once in a table about newly identified developmental neurotoxins (table 2 to be precise). It references only one meta-analysis on this topic.

The cited meta-analysis ("Developmental fluoride neurotoxicity: a systematic review and meta-analysis") which did a statistical meta-analysis of published papers and their data. They find a significant relation between the fluoride content of the drinking water and the IQ of the children.
However this is not undisputed (to say it mildly), since while a finding can be statistically significant, it doesn't necessarily mean that there is clinical importance. The other important point which @Doctor Whom mentioned is that a correlation doesn't imply a causation. So even if there seems to be a correlation between two things, they are not necessarily connected (see the famous example of storchs and birth rates). 

This paper ("Developmental fluoride neurotoxicity: clinical importance versus statistical significance.") is published as a comment on the original paper and claims exactly this. They say the the mean difference in IQ is about 7 points, which is not of clinical importance. Another paper critizes the relatively low quality of the original papers used for the study ("Fluoride and children's IQ."). 

They also mention a low consensus about what is a high concentration of fluoride in the drinking water and also show the very wide variation of the fluoride concentration in the original studies. The highest concentrations mentioned are around 10mg/l which is about 10x the concentration used for adding fluoride to the water. Even if we take in account that children are more sensitive to fluoride and also need lower fluoride concentrations to cuse some developmental damages (below the fatal concentration) this would still add up to quite absurd amounts of water they have to drink.

Regarding your questions: 


A developmental neurotoxin is a substance which interferes with developing neurons or nerve cells. It either inhibits their growth completely or at least impairs the cells.
Given on the articles I found (and cited above) I would say that fluoride is a developmental neurotoxin. Nevertheless it seems to me that it is not very strong in this context as the current studies are contradictory. Thinking about the amounts of fluoride present in water (even if the concentration is much higher than recommended) makes me doubt if this is really the causative agent.