Re: The 50th special will be feature length...
Which makes sense. It's the longest any production of nu-Who. The End of Time Part 2 is also 75 minutes but after that, Journey's End at 65 minutes and then various Christmas specials 2009 gap year specials at 60 minutes. And I guess 75 is still low enough that they could have squeezed it into the filming window they had.
"Internet message boards aren't as funny today as they were ten years ago. I've stopped reading new posts." -The Simpsons 20th anniversary special.