I too dislike this approach to "harden" a game. Creating large spawn groups, increasing creatures' HP beyond reasonable amounts, reducing the Medpack spawn etc. make a game more tedious, not more challenging. Challenge comes from superior enemy AI, better enemy aiming abilities, friendly fire in team-based games, increased potency of simple weapons for both you as well as the enemy, tighter time-limits for timed missions, etc.
I guess the single most important factor is enemy AI, something that has shown marginal improvement over the years. Allan, we seriously need you to get into the industry!