Wrack
Very fun oldschool FPS with a great cell-shaded look. For those into that kind of thing, the game feature Score Attack and Time Attack mode on all the levels, so that can definitely extend the short length for you. One-click download of people's replays from the leaderboards is a very nice feature as well, and I don't recall seeing it in any other game.
Wrack uses both a quicksave and a "limited lives" checkpoint system, however if you play on higher than Medium difficulties the quicksaves are only there to be able to quit the game at any time without losing progress, so quicksaving actually dumps you to the Main Menu. This means on Challenging and above difficulty you have to survive on those limited lives - and considering how many times I died on Medium during the playthrough, I figure that's not for me.
The game uses a kill chain system - killing enemies in a quick succession start's the kill chain, and longer the chain, the more points you get. Additionally, the kill chain has 4 "finisher" levels, and once it reaches one of them you can activate a weapon's finisher move, which has different affects depending on the weapon, and additional power per finisher level reached. These are all necessary to get high points, which is important for higher difficulties since you get an extra life every 50k points, but the more I tried to score high, the more I died :)
Despite the short length, the gameplay does get kinda repetitive. The only other minor criticism I have is that with everything being so colorful, it can be pretty hard to spot the small health, armor and ammo drops. I often had to run several times over the remains of downed enemies to make sure I didn't miss anything.