I wanted to expand upon the reasons I suspect removal should be worse than it has been (apart from when Brian Tinsman is lead-designing the set). This is a follow-up to How Expensive Should Removal Be? and The Magic Ecosystem.
Creatures are inherently powerful because they stick around, can keep you from losing and can win you the game. There's no doubt in my mind that's why Dr. Garfield and the Penn playtesters made them as weak as they were in early Magic, relative to spells. A couple of decades later, we now know that creatures were too weak and spells were too strong. We know this because, as fun as spells and spell-combos can be, the heart of Magic is summoning fantastical creatures to do battle for you. Yet the early years of the game were dominated by spells because the creatures of the time just couldn't compare.
Wizards has solved this discrepancy by making creatures better. The once-rare Serra Angel is now a mere uncommon, outclassed by Baneslayer Angel. Goblin Guide and Stigma Lasher are a bit more impressive than Jackal Pup and Ironclaw Orcs. Primeval Titan and his friends (the other four in his cycle, Wurmcoil Engine and Consecrated Sphinx) outclass almost every other six-drop by a mile. Have you seen Avacyn, Angel of Hope or Sigarda, Host of Herons? These are exciting cards, but they're not good for the game.
How can I say that? There are a couple metrics. First, if a card is an auto-include in every Standard deck of that color, it's absolutely too strong. A trading card game is about building your own deck and must-play cards steal part of that joy from players. Every card should be conditional enough that you need to think about whether it's really the best choice for your deck. Second, if the card automatically wins Limited games when the other players doesn't draw one of three valid answers immediately, no one will enjoy that game, including the winner (except for sadists, and even then, only the first few times).
The trouble is, how can any creature be good enough with cards like Doom Blade and Path to Exile running around? They could print a 20/20 for 2G and it would rarely do your deck much good in Constructed because your opponent will just kill it for less mana than you spent casting it. In order to make a creature relevant among such answers, it either has to be more resilient (hexproof, protection, indestructibility) or give you card advantage just for playing it, like the Titans. But invalidating removal just puts a premium on countermagic, discard, and land destruction — the three least fun lines of play in the game.
If we can't make creatures better (read: as good as the best spells ever printed), how can we remedy the disparity between card types? We can make removal and disruption worse. I think red removal is in the perfect spot right now. One mana makes Shock which kills X/2s, usually netting you one mana. Two gets you Incinerate and three gets you Brimstone Volley, which kill X/3s and sometimes X/5s, netting you 1-2 mana. Four offers up Chandra's Blaze, killing an X/4, netting you no mana, but 2 free damage. Six gets you Into the Maw of Hell, killing most any creature, plus a land. All of these answers are better, but only slightly than the questions they're solving.
Doom Blade doesn't scale like that. It kills any nonblack creature, regardless of size. So how do you make any creature that costs more than 3 relevant? My 8/8 for six is just losing me four mana on the deal. It's the same disadvantage as casting an aura on a creature and getting two-for-one'd, except instead of being down a card, you're down four mana. Which is a big deal. The only way to get around it is to make your 4cc+ creatures black, indestructible, hexproof or protection from black. Except Path to Exile still kills three out of four of those resilient creatures. So now only hexproof creatures are worth playing. But strong hexproof creatures are terrible for the game because they remove interaction, which isn't fun for anyone.
Effects that scale in cost, like Death Wind and Vendetta, work just fine, as do more limited spells like Tragic Slip or Assassinate, or more expensive spells like Iona's Judgment.
Not all Interaction is Fun
The other big part of the discussion is about interaction and the kind of interactions that are fun. Every creature and every spell with the word 'target' on is interactive. Runeclaw Bear attacks and blocks. Lava Axe interacts with your opponent's face. Stone Rain interacts with your opponent's mana base. Cancel is useless if your opponent doesn't cast a spell for you to interact with. Mindslaver's effect is entirely dependant on your opponent's deck, hand and board. How could you get any more interactive?
And yet, these aren't all equally fun effects. Attacking and blocking is the heart of the game. Creature combat, in combination with being a spell-slinging wizard and getting to build your deck however you like, is the biggest attraction to Magic. Lava Axe keeps the aggro player from conceding after her opponent has locked up the board. Stone Rain limits what spells he can play. Cancel prevents me from playing a spell altogether. And Mindslaver kicks you out of the game and forces you to watch as I violate your deck.
All of these effects are interesting and serve a role in the game. Apart from Mindslaver, I believe all of these should be printed at some rarity at some cost, even if it's more rare and more expensive than you're used to. Creatures are fun and should be worth casting. Disruption effects aren't fun and should be just good enough to prevent degenerate cards from dominating.
Creature removal is vital to the game. The biggest complaint from green players (before the last couple years) revolved around green's inability to answer creatures. There are a lot of creatures whose value is in some activated effect that never necessitates that the creature enter combat, and some creatures whose evasion means they'll never get in a scrap. That's good. Those cards add great depth and variety to the game. But every color has to have some way to deal with them. (This is why I love Prey Upon.)
As vital as it is, however, creature removal is not fun. There's a game called Milles Bornes in which you play a card on your opponent and they can't do anything else until they play the specific card that answers it. It's miserable. If you're lucky enough to already have the answer in hand, you lose your turn playing it and both you and your aggressor are back exactly where you started. If not, you just sit there drawing one card per turn until you get the answer. It often takes an excruciatingly long time to find and when you do, you spend your turn using it, and are back where you were in the first place, miles behind everyone else. There is no joy in "Here's threat X. Use answer Y or lose."
Rock-Paper-Scissors is a dirt simple game, but the philosophy behind it has powered countless excellent games because it's a great formula. Everything has a strength and a weakness, prey and a predator. You must guess which tool your opponent will choose and then select the one that beats it. As you add nuances, you might find relationships that add strategies like choosing the card that is less effective, but effective more often, or more effective, but more vulnerable. It's this underlying mechanic that makes Magic so interesting. My White Knight is better against your Sengir Vampire, but vulnerable to Shock and Nessian Courser. Your Shock is weak against my Nessian Courser and Sengir Vampire. But both of those are worse against your Unsummon than the Knight was. That's awesome.
Magic is infinitely better because there are numerous threats each with numerous answers, most of which don't knock you out of the game if you can't answer them quickly. Creatures can be answered by other creatures through combat (I keep repeating this, but it's vital: Creature combat is the star of the game), or enchantments, or artifacts, or planeswalkers, or spells. Planeswalkers can be answered by creatures (via combat), by other permanents or spells. And so on. But not every creature can handle every other creature, nor every spell every creature. The point is that there's a balance between everything-answers-everything and nothing-can-be answered-except-by-one-thing. That balance is what makes Magic interesting. Like Rock, Paper, Scissors or RPS-100, the fun is in predicting what your opponent will play and playing the thing that beats it.
Cards like Avacyn, Angel of Hope and Sigarda, Host of Herons are splashy and exciting, but they're not good for the game because they drastically limit your opponent's responses without giving up anything to balance for that. The Titans die to most good removal, but leave their caster ahead. And if you don't answer one of these or Consecrated Sphinx within a turn, your chances of getting back into the game are drastically reduced. The only strategic response to such an environment is to either play a mass of these unfun cards yourself, or else part of the unfun disruption trifecta of countermagic, discard or land destruction to prevent these bombs from ever hitting the table. All paths lead away from fun.
What I'm proposing is a game with good spells, good creatures and decent answers. A game where no choice is always right and where making crazy creatures battle each other is as valid as it is awesome. Getting there may be as simple as not making any creatures quite as bomby nor removal quite so efficient/universal. The jury's still out on Avacyn Restored, but surely you remember how much fun Rise of the Eldrazi was. Was there a Doom Blade or Path to Exile in that set? No. Was there a 15/15 with protection from colored spells and annihilator 6? There was? Er. Did it cost 6? No? Phew. Close one.