I'm going to throw out the names of three action games: Devil May Cry, God of War, Resident Evil 4.  These are all good games, but God of War has a very severe flaw that the other two don't.  God of War has the Gaming Repo Men.

In all three of these games you are constantly earning new moves/equipment with which to kill the demon puppets/monsters/not-zombies (that look and act a whole heck of a lot like zombies, but... aren't, supposedly) that are constantly harrying you.  By the end of the game, you've slogged through countless hordes of ever-tougher enemies, you've earned the orbs/orbs/pesetas to buy what you need to become a killing machine, and after overcoming that last nasty boss, what do you want to do?  Why, you want to start over with all your hard-won upgrades and power through the game like an unstoppable juggernaut, of course!

If you want the original experience, you can always start a new save, but the reward for the upgrade-purchase action game is getting to blow off steam by being godlike your second time through.  It's an unspoken part of the deal, like the fortune cookie at the end of your meal in a Chinese restaurant.  The menu doesn't have to say "stir-fried vegetables and and noodles in brown sauce, served with white rice and a fortune cookie," because you know you're getting the fortune cookie.  It's a given, like getting the cup with your drink instead of having them pour iced tea on you.  Devil May Cry understands this.  Resident Evil 4 understands this.  Hell, even a hopelessly mediocre game like Van Helsing understands this!  God of War does not understand this.

In God of War, between the time you defeat Ares and go to start a new game, the Gaming Repo Men come and take away all the moves you purchased.  All your magic is thrown in the truck and taken back to the store.  They don't offer to negotiate payments, they just haul it all off.

Now, I'm singling out God of War here only because was the most recent game to disappoint me that way.  After I beat the game, I played with the Challenge of the Gods a bit, then I started a new game, because I wanted to see the enemies at the beginning of the game get vaporized by my fully-upgraded might.  Instead, I had to constantly stop myself from trying to bust out moves that I used to have, but didn't anymore.  This has happened with other games in the past, just most recently with GoW. 

On the other hand I finally finished Resident Evil 4 recently, started a new game, and ran into the pueblo with my fully-upgraded Striker and my Red9 that is basically now a small, extremely accurate shotgun.  They all died.  Very quickly.  It was great.

One of my two DS games is Episode III, a great beat-'em-up with an upgrade system.  Guess what?  Yup, you can replay any level with all your upgrades.  Even a beat-'em-up, traditionally a play-it-again-exactly-the-same-way genre can get this right.

So my message to developers: don't take my stuff.  If I put in the time and effort to get all the orbs/crystals/bolts/potted-plants to buy all these upgrades, let me actually have them.  Either that, or stop using the word "buy" on the upgrade screen, and start using "rent."  And make sure to proclaim upgrade rental as a feature of the game, to facilitate my not playing it.