Some decks like Freeze Mage, Miracle Rogue, Aggro Shaman, and even Tempo Mage to an extent have so few threats that it's relatively easy to check the few threats that they do generate. They win mostly through types of burn and burst damage. If you can survive their burst, you survive their deck. Other decks like zoo or aggro paladin run a whole bunch of little wienies and run out of gas quickly if the wienies are answered early. So essentially, these decks have a "finite" amount of damage, and realistically can never claim to have infinite damage. Their threats are few or tiny.
Midrange decks run enough meaty threats that they cannot all be answered because generally, nobody can actually run that many answers. So they don't really have finite damage. A midrange decks looks at Reno, says "that's cute, grats on buying a turn or two, but you still have to answer the fundamental problem, which are my threats on the board". If you are actually playing a midrange deck against a Reno deck you really don't have cause to complain. A midrange deck should be able to close out a game much earlier than "forever", even with a Reno heal.
