Minor spoilers for assorted games follow.
An overly-difficult end-game is one of the worst god damn things ever. I
love end-games when they're done well. Metroid Prime 3? Awesome. Nice and fast-paced, gave you powerful weapons, made you stronger while still maintaining a constant feeling of "threat". Half-Life 2? Awesome! Fast! Gave you a ton of armour and a ton of health and a super-powerful weapon, more than compensating for the hordes of elite troops thrown at you. Mass Effect 2? Awesome. Again, fast-paced, made you feel totally in control. Then take, say... GUN. Not an RPG - I seem to suck at giving RPG examples in the RPG thread, but

it, it's a great example of this, so we're using it. Slow as a mother

er, and the final boss made you feel like a total wuss.
So... what?
A good end-game is two things. First of all, it is tightly paced. The first three examples? Fast. GUN? Slow as a cannon mounted on a railcar (If you can see what I did thar.). Making the endgame the absolute hardest part is a great way to slow it right down. Nothing kills momentum like having to reload.
Second, a good end game makes you feel... well, it should make you feel kinda badass. "

yes! I just performed an abortion on a
planet!" "

yes! I am an
awesome squad leader!" "

yes! I just killed five people by throwing the corpse of one of their
teammates at them!"

yes, I just took out Sephiroth like

in' nothin'.

yes, I just destroyed Mother Brain like an angry momma bear.

yes, I finally showed that bastard Vizier who's boss.
Not, "Oh, Christ, this son of a bitch just ain't going down!" That's what Meta-Ridley is for.
EDIT: Obviously, this is not a universal rule, and I make exceptions for certain games. Like I Wanna Be the Guy. Holy balls, The Guy is a bastard. He can block bullets with his
eyebrows.