When running a 1 shot, don't have enemies with high defense or defensive capabilities. The longer and less exciting the fight the less players wanna play. I recently ran into my players in my low fantasy play test campaign loosing some of their gusto when faced with enemies they had to roll rather high to hit (AC 20 vs level 4 characters). This morning we where playing a Savage Worlds 1 shot and the enemy had "hearty" which negated the "2 stuns = a wound" roll. We ended up rolling dice and stun locking it for about an hour or more...
In short- just give players a quick thrill. High attack enemies are great for this, multi-target enemies are nice too (I think 4th ed calls them "solo monsters"). Toss in a bunch of mooks, toss in a maybe one significant challenge at the end. (BBEG* :D) Still- remember players dislike discouragement (like 99% of the world) so low defense is advisable. Remember- they want to win.
Just a quick note based on observations of recent games I've been playing it : )
*BBEG stands for "Big Bad Evil Guy" aka "the boss".