Nothing, in my opinion. If it's not accepted, I could imagine that if this sentence is in a lesson about translating "there is/are ...", they might want you to go for "there is" in DE-->EN as well.
I guess "A dog is standing in front of our window" might feel a bit verbose in English, where (unlike in German) you can simply say "There is a dog in front of our window", but - not being a native speaker - it seems like a sentence I might encounter in a novel (while I wouldn't use it to point out the dog to my family in real life).
there is = fixed expression es gibt if you talk about the mere presence / existence of something
there is = location à la da ist / sind etc. if a place is given
There is a house in the woods. = In den Wäldern ist ein Haus.
There are people there. = Dort sind Menschen.
There is a phone, too. = Es gibt dort auch ein Telefon.