Heroes of Might and Magic 4. What REALLY is unforgivable in the game is the AI, which - mostly due to time pressure - even its own developers described as "lobotomized". But it has many things new to the serries, including obstacles affecting line of sight in combat, towers that need to be manned (no more turrets that are either too weak or too strong), separate initiative and speed stats (greater flexibility in units). Gone is the notoriously hard to balance Attack Skill, Defense Skill, Spell Power and Knowledge (attack/def got stronger and stronger as turns passed). Heroes4 riped off Master of Magic spell system, which in turn ripped off M:tG spell magic colors. This means that magic schools are actually unique and work in different ways. Heroes 4 is a broken game in a couple of ways, but a library of intriguing mechanics as well.
Nox. Easy to mistake for a Diablo clone, Nox by Westwood Studios (neglected by EA) has no random loot, no chance-based dodge or shield block or hit chance, practically no resists. All spells remain useful thorough the game. Nox has many interesting spells and mechanics. It is skill-based instead and combat is very satisfying. The most noticeable thing in the game is its fun line of sight algorithm. In Diablo, you can anticipate big fights because you can see past walls. Here, you can't. Teleglitch is a new game that also has this mechanic.
Age of Wonders: Shadow Magic. Like Heroes 4, it was sunk by bad AI I believe, but is a great game especially in multiplayer. I would describe it like "Heroes 4 done right". Flying units, spells on adventure map, Master of Magic-like research and magic system, the ability to build towns and roads as well as raze&rebuild neutral structures. If an ice witch passes over land, it's permanently covered in snow. Try that over water or enemy city's crops !
And play board games. Board games have many mechanics unseen in computer games. Try ones like Dominion, Neuroshima Hex, Seasons, or Dungeoneer. The last one may appear like just an improved version of Talisman, but it has one wonderful mechanic. Buffs and positive powers are used from a SEPARATE pool. Normally multiplayer games have a problem where by spending your resources on hostile actions you don't help your cause but make the target of your malice easier to beat for others.