I hate blank-slate main characters, and here's why.
These days, most games have blank slate main characters. The idea is that the player's avatar is so incredibly nonexistent (especially outside of cut scenes) that the player can fill him with any personality he chooses.
To say this is common is an understatement. It's ubiquitous. To the point where it exists even in games with seemingly strongly defined main characters.
For example, in Gears of War, you play the improbably named "Marcus Fenix". He seems like an actual character. He's got a badass voice actor, he has lines of dialog, he's definitely not visually neutral.
But he's still an empty character. After establishing his very basic personality and history in the early cut scenes, he rarely says anything more emotional than "keep moving" and "could be a trap".
This sort of thing is easy to see if you compare the main character to the secondary characters. Compare and contrast: your main character in any Bioware RPG. Even if we take into account all the dialog you choose (and we shouldn't, because it doesn't actually constitute the avatar feeling any emotion), you STILL have less character development dialog than any given party member's left big toe. The other characters will have strong judgments, wacky opinions, fun lines of dialog, actual emotions on their faces... you, on the other hand, get to choose whether to kill the beggar or give him money.
Obviously, you can have good games with hollow avatars. The world is full of them: the Final Fantasies, if you like them. System Shock II. Fallout I & II. Half Life. However, this long list is not because of any strength in the idea of a hollow character: it's because there are ten times more hollow-character games than defined-character games. Even games that theoretically have strongly defined characters, such as the Spider-man games, end up hollowing out the character for your use.
But compare these to games which have a main character that is not afraid to have a strong personality. Psychonauts. Beyond Good and Evil. Planescape: Torment. Sly Cooper. Grim Fandango. Most of the best adventure games (and the worst, I admit).
These games have a very different feel from hollow-avatar games of the same sort. The main character's existence as an actual person shapes the whole game. It shapes the plot, the gameplay, the color, the EVERYTHING. You are living in their world.
The hollow avatar started back in the beginning, when you didn't really have much choice. Even if you made your avatar a bright red and blue clown, you couldn't really show him doing much in terms of emotion or characterization. But these characters were not intended to be hollow. They just HAD to be.
In the game designers' minds, the characters were fully formed. You can tell by the way the world is so strongly shaped around their personalities... even though you can't actually see their personalities. Little Nemo and Bonk are very clear examples: you don't really see much in the way of emotion or personality during the normal course of the game, but the whole world is built around their very strong personalities.
In some games, hollow avatars are used to great effect. System Shock II being an excellent example, or Half Life if you prefer the wussy little brother. These games make use of the same basic limitations: you never see your avatar's emotions because A) you never see your avatar and B) your avatar never runs into anyone he can have a chat with.
Similarly, Ico and Shadows of the Colossus both have hollow main characters, but, again, they are hollow because their personality rarely gets a chance to show. The worlds and plots are built around a very strong personality and set of emotions, and this shines through.
That's not the case in a game like Gears of War, where you spend the majority of the game staring at an armor-plated football player hanging out with other armor-plated football players. Obviously, your avatar has a billion chances to show emotion and personality. You can always see his body language, you almost always have people within talking distance, so why is it that the other characters all have interesting lines and you always just grunt? Why is it that the OTHER characters express all the things I, as THE PLAYER, am feeling?
Even in games where you make your own character, such as Oblivion, I think I would enjoy playing someone with a personality.
I think people assume that a hollow character is the best thing because it allows anyone to give any personality to the character. But that's stupid. If you've played Beyond Good and Evil, which avatar feels more real? Which avatar adds more to the game? The photo realistic, extremely cool Fenix... or the cartoony, slightly insipid Jade? I'll give you a hint: it's Jade. If you doubt it, go play both games again.
In honesty, I would have preferred to play Cole or either of the Carmines in Gears of War. I have a feeling I would have felt the war far more personally that way.
It's not as if a hollow character can be filled with any given personality, either. Even if we're playing something like Oblivion, with an almost unlimited set of options, the personality we give our character is going to be tightly linked to how the character plays. A good example is the assassin: a lot of people wanted to be assassins in Oblivion, but you can't make your avatar feel like a dangerous assassin. A) Assassins have to frolic (literally) through the woods picking daisies and B) bows suck. The "dangerous assassin" personality dies quickly, mutated into something bizarre and perhaps hilarious.
To me, this means there is no excuse not to give the avatar a strong personality. Make it permeate every facet of the game.
"But what if the players don't like the main character?"
That actually happens pretty rarely in decent games, and the reason is because the quality of the gameplay and plot will change their judgment on the quality of the character. Jade's character is actually quite irritating, taken out of context. But the gameplay is good and the way she's exposed to plot points takes the butter out of the smarmalade and lets us admire her.
Sly Cooper's about as interesting as a piece of cardboard, but he's got an extremely STRONG two-dimensional personality, and the rest of the game supports it. The main character from Destroy All Humans is practically ONE-dimensional but, again, the game makes him interesting to play as.
So... characters. Yeah. That feel things. And have personalities.