Well, I prefer texts that do not obstruct the video. The borders should be thin and light. Anything with thick borders and shadows is just horrid, sorry to be break the first post. Here is it should be:
1. Thin and Light borders, not too thin.
2. Fonts should be legible and thick enough but not too thick (follow number 1) -- not curly such as TrebuchetMS, any Serif, One Script, and Dom Casual... those fonts completely kill the video. Use styles such as Arial (Bold), Tahoma.
3. Font size should not be too big.
4. It should be bottom-aligned on the right place.
A good guideline for #3 and #4 is to treat safe areas (action safe and title safe regions). Action safe has a margin of 5-6%, and title safe has a margin of 10-12%. Consider a one-line subtitle, your subtitle should start at 6% from the bottom of the video and end at 12% from the bottom so that they don't disrupt the video. See the following picture
(From wikipedia)
Therefore:
For 720p, subtitles should have a vertical margin of 43 pixels with a size of 43 pixels with both left and right margins of at least 87 pixels.
For 480p, subtitles should have a vertical margin of 29 pixels with a size of 29 pixels with both left and right margins of at least 58 pixels.
Note, for two-liners, they will fall on the title safe area, which is desired. If you can get all of your subtitles one-liners, you can push all of your subtitles to the title safe areas (i.e. 12% margin)