How to get rid of whitespace between Runs in TextBlock?

The spaces between the run tags cause the spaces, this is the easiest fix.

<TextBlock 
   HorizontalAlignment="Center" 
   VerticalAlignment="Center"
   FontSize="10" 
   FontFamily="Arial" 
   Foreground="#414141">        
      <Run Text="{Binding LoadsCount}" /><Run Text="+" /><Run Text="{Binding BrokerLoadsCount}" />
</TextBlock>

Because anything between the <TextBlock> and </TextBlock> is targeting the text property of the TextBlock the whitespace from the breaks between the runs causes the effect you see. You could also shorten it to this.

<Run Text="{Binding LoadsCount}" />+<Run Text="{Binding BrokerLoadsCount}" />

This MSDN article gives all the specifics on how xaml handles the whitespace

http://msdn.microsoft.com/en-us/library/ms788746.aspx

If you were curious why a break and a ton of tabs translates into a single space

All whitespace characters (space, linefeed, tab) are converted into spaces.

All consecutive spaces are deleted and replaced by one space

Another option is to comment the space between Run tags, maintaining the code readable and removing the extra space.

<TextBlock HorizontalAlignment="Center"
           VerticalAlignment="Center"
           FontSize="10" FontFamily="Arial" Foreground="#414141">        
    <Run Text="{Binding LoadsCount}" /><!--
 --><Run Text="+" /><!--
 --><Run Text="{Binding BrokerLoadsCount}" />
</TextBlock>