I think games ought to be rated in a similar way: Plants vs. Zombies would be graded as Easy and VVVVVV would be Hard Very Severe. These aren't knocks on the games anymore than calling a walk in the park easier than a walk in the Himalayas: it's the truth converted into a subjective ranking system. If you think Sonic is just the drive-in place where servers wear roller skates when they serve awesome drinks, you are going to get stuck trying more difficult games. If you want want to hone your competitive Starcraft skills, the lower end of the spectrum won't help.
Obviously there need to be other axes with which to evaluate games such as Entertainment Value, Innovation, Addictiveness, Story, and um, Graphics. (I like good-looking games as much as the next guy, but it seems like there's often an inverse relationship between entertainment value and graphics.) But difficulty is often the primary limiting factor in whether a player will enjoy a particular game or not. Publishing accurate (if not precise) ratings would solve a lot of problems with people being exasperated by surprisingly difficult games. Like, for instance, John Walker's experience with Metroid Prime.
I should be quick to point out that even though I've completed the first two Primes and am on the cusp of the final assault of Corruption, I'm not saying that Mr. Walker is bad at video games. In fact, it's quite likely that he's better at games than I am, but like Edmond Hillary, who had learned the lessons from numerous other explorers, I have many little advantages that are worth more than the sum of their parts. (More on this anon.) Rather the game failed him by not letting him know that it has Hard Very Difficult boss fights.
After the various story-laden introductions, the games settle into an easy-going exploration mode. Sure there are signs everywhere that something has gone wrong: poisonous pools, a dark, life-stealing mirror world, and a creepy, disease-like corruption. But these are offset by a series of organic, lived-in environments. It's telling that I continued to avoid harming the more peaceful creatures even after I found out that they continually respawn in the same locations. For the most part, you have limited ability to alter your surroundings and that makes them seem more real somehow. (And when you do make a permanent change to the world, it seems so much more monumental.) You can blatantly slaughter a whole herd of peaceful Zoomers, they will return to their proscribed paths the next time you travel through the area.
For the vast majority of the game you explore until you find your way blocked by some obstacle, search for the suit, weapon, morph ball or visor upgrade that overcomes the obstacle, defeat the boss that guards the upgrade, and go back to the areas that are now open to you. It sounds artificial and it is. Almost every location is a room with at most three doors or a hallway. On the other hand, you generally don't notice the linear space because Retro Studios crafted the illusion of space "out-there". Exploration is the core of the game and for the most part, the boss battles are more like puzzles (that need to be explored) than grueling, multi-part slug-fests. Those usually come near the end of the game, but some of the hardest fights for me are the very early bosses that you have to face before you've gathered enough upgrades.
It turns out that three classes of upgrades are not required explicitly to make progress: energy tanks, power bombs, and missile capacity. These tend to be better hidden and/or more challenging to reach than other upgrades so there's a sense in which they are bonus rewards for those who enjoy getting 100% completion in games. You'll still run across enough of each upgrade to avoid getting killed by many of the mid-game boss fights, so a lot of players will just be inclined to skip the harder to find or reach ones. But this is a mistake because you'll want everything you can lay hands on to take on the final few bosses. To go back to the mountaineering metaphor, it's like starting an assault on a peak without enough rope, pitons and carabiners.
I mentioned before how I had some advantages over players of the GameCube version of the first two games and one of them is the wealth of tips and strategies for finishing the games which are now freely available on the internet. Little things like knowing which beam or missile causes the most damage to a particular enemy or knowing where an elusive upgrade can be found make huge differences in the end. There's no shame in looking for tips when you get stuck, in my opinion.
The other set of advantages comes from changes to the games in the Trilogy edition. Aiming with the Wiimote is probably the most intuitive input possible for shooting games, if not quite as accurate as mouse and keyboard. I gather all the games on Normal difficulty are easier than they used to be. Once you get morph ball bombs in the first two games, you also get the spring ball ability that makes little hops much easier than always doing bomb jumps. Oddly enough, having all three games available in one place helps since if you get stuck on one boss, it's easy enough to switch to another game. At one point I was stuck on all three games (on Meta Ridley, Bomb Guardian/Alpha Sandigger, and Mogenar for those who care) and simply by switching from one to another I got past them. It helps to clear the palate and avoid banging your head against a hard surface to see a different difficult part of some other game.
It's a bit unusual from me to feel compelled to finish a game, but I am compelled to finish each of the Metroid Prime games. Unlike Super Mario Galaxy, which is structured as a series of increasingly difficult challenges, the Prime games are structured as preparation for a final challenge. The onscreen result of beating the final boss turns out to be a not-very-interesting1 cutscene and roll the credits. But the offscreen result was more satisfying. Completing a difficult challenge after spending so many hours working toward it provides the same sort of intense satisfaction you might get from hiking to a great view. Skipping straight to the cutscene just can't have that effect any more than driving to the top of a mountain can match walking there.
Retro Studios built up an amazing reputation on the backs of these games and for good reason: they are finely crafted masterpieces. Everything about them is just so well done from the the controls to the music to the menus to the item descriptions to the color scheme. And given the chance to improve things in the Trilogy version, they did from what I've read. The star of the show has to be the environment you get to explore, which is universally and rightly praised. Trying to explain it is sort of like trying to tell someone why the Grand Canyon isn't just a big hole in the ground. You just have to see it for yourself.
1. I've seen the various endings on YouTube and none of them are especially amazing. Ok, so you can see more of Samus' character model and hints of future episodes if you gather 100% of collectibles and scans, but who cares?