Looking at a Random Card: Swarm of Rats
(What am I doing here? Read here!)
Swarm of Rats, first published in Portal, was WotC’s third try to get across the flavor that one rat was merely an annoyance, but a swarm was dangerous – the bigger, the more. (Their first try was Plague Rats in the original Magic edition, their second try the abysmal Pestilence Rats from Ice Age.)
I see here largely a development failure: This card, although already an improvement on Pestilence Rats, is just too weak. On its own, it’s nearly useless. With another rat at its side, it’s still underwhelming, with several clearly stronger options available. Needing two more rats on the battlefield (under your control, to be precise), is already quite a tall order – and then this card is STILL unexciting as a 3/1 for 1B, although at least not clearly outclassed by superior cards. At any cost higher than B, its toughness of 1 is too much of a liability. (I see, however, that a cost of just B might have pushed the card too far.) Being forced to focus on a creature type is a restriction which needs to be rewarded a lot more than Swarm of Rats does.
From a limited point of view, this card COULD be fine IF put into an environment already featuring a strong rat theme (including better incentives to “go rats” than just the Swarm). If the Swarm would more or less reliably be 2/1 and often larger, it would make a fine 2-drop in a rat archetype. However, despite a few other cards supporting the rat theme (Ratcatcher, Marrow-Gnawer, and those narcissistic Relentless Rats), there isn’t enough to go with even if you create a limited environment by yourself.
If rats ever get more tribal love from WotC, there will be a place for Swarm of Rats in limited environments as well as possibly in casual constructed. However, it will never incentivize you to build a rats tribal theme, only help filling out an existing one. As things stand now, this card is mainly useless for all purposes, and it has to be rated with the the existing card pool in mind. Being moderately flavorful doesn’t help it much. I give this card an E (with the option to go up to D or even C, if future releases support its theme).