In no particular order:
1. Over The Edge
No levels, self-expression? Well, the system is so laughably large-grained that there seems to be very little difference between characters even. Whereas in D&D two first level fighters might have very different abilities and feats, some people argue that OtE characters are all pretty much the same. But they'd be wrong on two counts.
In D&D, with the emphasis on combat and, particularly at higher levels, destructive power, the bottom line is about how much damage you can do in a round. If you play a PC whose focus is not destructive power, like rogues, in a mixed group you soon find yourself with little impact on the game because fighters and mages are so much more powerful. So players tend to maximise character improvement so that their PCs are always on the upper boundary of death-dealing power. The main differences seem to be about range. Sword, bow, or fireball are the options, or short, medium and long.
In OtE, two fighters are likely to have 4d in their main fighting ability. Or in other words, they are optimised to deal the most damage possible. The differences however can be extreme. One might be a "reluctant pugilist" and the other a "bullying streetfighter". The weapons might be the same, fists, but the self-expression in the description not only makes the characters sound more interesting but also means that the character must be played in a certain way.
This is probably slightly disingenuous. The D&D feat trees and multiplicity of classes do allow for more self-expression as the character develops but this is always much more constrained than the OtE method. And OtE is much more simple to learn than D&D. The rules are shorter than D&D's index.
OtE also has a cracking, if mad, background and some of the best help for GMs.
2. Call of Cthulhu
I don't like this game's system, Basic Role Playing. It's inconsistent in application, full of patches and does nothing to support the kind of game play that most Cthulhu players I've met seem to like. So why do I like it? There are two main reasons, the great background and the powerlessness of PCs.
Lovecraft's (or perhaps more properly Derleth's) Mythos is one of those great literary inventions where the authors did just enough to create a mystery and give a good sense of flavour without fully fleshing out the background so that it is an ideal vehicle for roleplaying. With Lord of the Rings, so much of the history and mythology has already been filled in by Tolkein that you find your self scratching around for scraps of untouched wilderness in the books to make your own. With Lovecraft it's the opposite. So little actually describes the plans of the mi-go or the raison d'etre of Yog-sothoth that there's acres of space for the aspiring GM. Even if you make the PCs children's TV puppets and the deities large stripped stuffy cats, it still works!
In D&D, if your PC has high stats and good weapons, your likely to win through. In CoC, this actually matters very little. Sure high power might mean you go mad less quickly than your friends. But if they are all climbing the walls with pockets stuffed with dynamite then you probably don't want to hang out with them anyway. In effect, the background and the SAN mechanic nullify all the pointless gumph in character generation. I forlornly hope that if mechanics and background worked together the game would be a world beater but I've come to realise that the scrappy system is what attracted many of the players in the first place although it is probably the background that kept them coming back for more. Gamers are rather conservative for the most part and narrative enhancing systems seem to frighten many. Simulation is seen as a safe option even when it's pretty pointless in application, as it is for this great game.
This powerlessness also sets up the conflict for the players. Although not always voiced, there is usually a subtext of how far characters are prepared to go to stop the secret menace of the Mythos and it's not uncommon for characters to fail. Nor is it uncommon for them to come into conflict with the wider society something that rarely happens in other games where the homicidal tendencies of characters seem to be the norm. Sure I've nothing against a simple, or even sophisticated, dungeon bash but I don't find much long term sustenance in that form of gaming.
As the main settings for the game are set within the last 120 years, it is relatively easy for players to have more realistic characters than it is within a pseudo-medieval game. Self-expression thus is supported by the easy to use background rather than challenged by any differing notions between the players of how characters might act in a given situation.
And although Lovecraft was not a great writer, his nihilistic fiction hits a nerve and the Mythos oeuvre is ever-expanding. There's probably more fiction that directly supports Call of Cthulhu than any other game. This is not a Glorantha or an Empire of the Petal Throne where background reading is almost a prerequisite to the game. This is here and now, and if you don't do something about it, you and what you care about will be squished by some cosmic juggernaut whose road just happens to cross your path. Call of Cthulhu, the game of heroes!
Even Delta Green, the post-modern version of the game, works well. Here the heroes are set up against people who wish to make some profit from their contact with the Mythos. Well, it's not really po-mo, it's probably more like the conflict between different brands of capitalism that is the American political scene. The mainstream is in bed with the Mythos but is heading to hell in a handbasket. Delta Green is the only voice of sanity, i.e. Ralph Nader. Guess who wins the argument, and who wins the war?
End of Part I.