Watery batch
I made a batch of yogurt last night but it turned out watery. I added the starter slurry when the temp of the milk was 89 degrees. That's the only difference I know of. The incubate temp was 93 degrees. The incubation period was about 11 hours.

  Tastes good but slightly lumpy and slightly stringy
I made a batch last night using skim milk and plain, no fat room temperature yogurt (2 tbsp) as a starter. Incubate temp was 89 degrees, inside an oven using the interior light. The taste was great.

  Tastes good but stringy
I made a batch last night using 1% milk and plain, no fat Greek yogurt as a starter. I used 1/2 gallon milk, incubate temperature was around 91 degrees, incubate time was 11 hours. I heated up the milk to 180 degrees, let cool to 105 degrees. I made a slurry of starter yogurt and cooled milk and mixed in. The yogurt tastes great but is stringy. I did not have any dried milk to add.

Made again last night. Used oven/oven light as incubator. Turned out wonderful.

I used a gold coffee filter (i.e. not paper) for filtering the yogurt. After about an hour, it was nice and thick.