The single player story in The Frozen Throne (TFT) progresses much like that of Reign of Chaos (RoC). You start off as the night elf warden Maiev (one of the new heroes introduced in the expansion), who is hell-bent on eliminating Illidan, the allegedly evil brother of Furion Stormrage, the noble Keeper of the Grove who protected the Night Elves from the Burning Legion in RoC. The story then takes more twists and turns than a soap opera, and jealousy, betrayal, and misconceptions abound.
The TFT storyline is complex (like many games these days), but solid. Even so, it falls a bit short from the pinnacle achieved in Starcraft where the detailed characters really drew us into the story. In Warcraft however, we only get two types of characters, noble and evil, and hardly any are human. All the main protagonists are brave and willing to die for their cause, and very few have character flaws that extend beyond the usual power hungry or vengeance-oriented cliche. TFT is still better than the vast majority of RTS storylines, but it fails to live up to past Blizzard classics.
The single player campaign plays out a bit differently as well. Many of the missions concern moving groups of characters from point A to B, and there are only a few that actually force you to build a fully functioning base from the start. These are almost RPG-like in nature, and while some might enjoy this, others will want their RTS to play more like strategy game, with huge battles and full base management. But one bonus is that by starting off with few troops, you work hard to keep them alive, thus increasing your micromanaging skills increases substantially. This naturally pays dividends when carried into the multiplayer arena.
In terms of graphics and sound, TFT blends perfectly with the original Warcraft 3 engine. The new units all look as though they've been there from the beginning and there are no telltale signs (a la Diablo) of being a quick-add from an expansion pack. The music has changed and is more ambient than the first, while shifting to a colder tone which integrates well with the expansion pack's title and motif.
The highest compliment one can grant an expansion pack is how different it can make the experience, while staying true to the original's look and feel. TFT does just that, and makes improvements on the RoC expansion. Experienced gamers found multiplayer RoC games quite predictable, as players developed the three races into a standard setup for each session. The game, in short, became a tad boring for devoted players, and many took a leave of absence until Blizzard shuffled the deck. TFT now eliminates the "one strategy beats all" issue, and prompts players to utilize a multitude of varied paths to victory.
The most obvious change is the new unit additions to each of the races. Each race now has at least two new units, which address specific weaknesses found in RoC. It was common knowledge that Orcs had poor air defense, so TFT introduced the Troll Batrider, a flying version of the goblin sapper that can detonate and deal huge damage to other flying units. The Undead ghouls were next to useless late in the game because of their low health, and that's where Obsidian Statues come in. These heal the units automatically using their virtually bottomless pool of mana. The Night Elf race, while fast, lacked a unit able to withstand a lot of hand-to-hand combat. Enter the Mountain Giant, a huge, lumbering Stone creature that can not only take a tremendous amount of punishment, but can dish it out as well. The Humans get the Spell Breaker, an elfish magician that steals enemy units' enchantments to cast on its own.
In addition to units, each race also receives a new hero that also bolsters specific weaknesses. The Undead's inability to pull off an early game rush has been remedied by the Crypt Lord, a mobile bunker that can summon attack beetles from corpses. The new Night Elf Warden comes equipped with the race's first Area of Effect spell, allowing the female warriors to do substantial damage to a cluster of opposition. The Orcs certainly have stamina, but wore down with each engagement and had no effective way of replenishing health in the early game. That is a thing of the past, and the New Shadow Hunter uses healing spells that can chain through a group of units. Being susceptible to any kind of hero spells, the Humans acquire the Blood Mage, which can siphon mana from any enemy unit to contribute to his own mana bank.
One major change that directly affects the gameplay is the neutral heroes that can now be recruited at a tavern. At about the same time your altar is up, you can send any unit to a tavern and recruit one of five heroes. The heroes all vary from summoning unit casters to melee masters, and this wild card throws off the idea of a set strategy, and allows the player to customize his hero (the most important unit of an army) and adapt to what the enemy is doing.
Once you add in minor changes like the new race-specific Item Shops, the new towers and the upgrades to existing units from RoC this creates a gaming environment that has completely turned upside down. Everything from the early setup, and well into the late game, has been restructured and allows the players much more flexibility to adapt and counter what the opponent is brewing. No longer are you stuck on one technology path. TFT encourages (and sometimes forces) players to branch out their strategies and experiment with new unit combos.