Basically, I love 4e monster design. However, it's important to "get it right" in two ways.
1. Make sure the math works. High level monsters are pushovers in 4e even after a lot of retooling.
2. Tie the numbers to some narrative reality. Okay, you want an orc warlord to challenge your 10th level party, so he oughta have X HP, Y AC, and do Zd10+Q damage. Why?
Don't just slather on HP to humanoids like you've got a trowel of meat. Make a HP mean something other than just "you have to stab him more times" (while also making it so you do have to stab a huge dragon more times).
Don't just boost defenses "because he's higher level." Either provide a list of templates in a sidebar, or make it so AC doesn't have to scale so fast, so it's okay to have an orc warlord with AC 10. If you go the first option, have the sidebar include three columns: AC, lightly armored, heavily armored.
For AC 15, maybe the Light entry reads "unarmored agile enemy, mage with magical shield, or leather armor and shield." Heavy reads "chainmail, hastily-donned plate, or bear hide."
AC 25 for Light might read "supremely agile enemy in enchanted mithril chain, mage who flickers out of existence to dodge attacks, or masterful swordsman who parries and deflects attacks with unparalleled skill." Heavy might read "enchanted plate and shield, armor crafted from the scales of a draconic demigod, skin transmuted to adamantine."