I have a question, I've been riding today on a very steep uphill road with my friend and I noticed that, when I pushed my bike into the 1st gear and tried to gently sit on the saddle (I've been standing all this time as I attempted to finish the climb), my front wheel rose from the ground some 15cm and I almost fell on my back! It kind of freaked me out.
Anyway, my friend said it never happened to him.

I have a pretty large bicycle (GT XL 21.4" frame /75mm stem, I'm 187cm /6' 1 5/8" tall), so could that possibly be the cause? And while I'm here, are there any other technical disadvantages with riding a pretty large MTB?