What are some of the main differences between German and English?
They are two different languages, with different grammar and different vocabulary. Speaking of vocabulary German's is mostly Germanic while English's is partly Germanic but has many foreign Romance (mainly French and Latin) words.
I know that German has gender and English does not, and that German has the fancy umalauts and such, but other than that, what are the differences?
Different vocabulary (see above), gender, and noun cases. The umlauts are just orthographic conventions and German also has the ß. There are probably more differences that I don't know of but I'm sure someone else here can give you a more complete list.
And WHY did English get rid of them?
English never had them so it never got rid of them... It did however have æ, þ, and ð which it has gotten rid of over time.