One factor that makes or breaks a monster movie, whether it's vampires, werewolves, or aliens, is consistency, and what I mean by consistency is this: are the monsters as easy/difficult to kill at the start of the movie as they are at the end; do their powers stay the same?
I just finished watching the fourth installment of the Underworld franchise, and I grumbled the entire time. One Lycan (werewolf) would take a silver bullet to the heart and flop dead. Another Lycan would be shot in the same spot, as well as take several other rounds in the chest, and manage to sprint off. At one point, the main villains said they'd been taking injections to be immune to silver, yet when it came to the end of the movie, the largest, baddest Lycan of them all was killed by a carefully planted grenade containing silver.
I have to say, as much as I enjoy Kate Beckensdale in tight leather doing anything, I couldn't for the life of me figure out her abilities. One minute she ran from anything with a gun, needing to be saved; the next minute she moved faster than light and killed an entire hall full of armed guards in a single attack. Why did she move that fast only once? Why is her aim only accurate a second before her or someone she loves is a breath away from dying? Either she's skilled and powerful enough to take out rooms full of guards, Lycans, and vampires while they tote all manner of weapons or she's not; it can't be both.
As for movies involving aliens, I recently viewed Battleship. It was a prime example of my largest pet peeve--aliens that landed with immense power at the beginning of the movie yet died easier and easier as the film progressed. Majority of the alien invasion stories I've watched in the past decade have been plagued with this flaw. Humans get the floor mopped with them for a little over forty five minutes, then miraculously discover a chink in the armor (sometimes, that's literal) and start laying waste to aliens in droves.
In Battleship, rockets that did nothing to the alien's rolling balls of destruction suddenly blew them to smithereens. In Battle: Los Angeles, alien armor that took six guys firing machine guns to penetrate were suddenly shredded by a single marksman, hitting the same spots. Overlooking the obvious fact that an alien species would probably adjust to enemy tactics as fast as humans, if not faster, the humans never upgraded their weapons. Therefore, whether it's five or fifty minutes in, it should require the humans the same amount of bullets to get through the alien's armor.
That's just common sense.