Here's a quick breakdown...

Both Runebound and Wow: TaG are about taking a hero and building them up. The process of building and strengthening your character is more or less the same across both games: explore, fight, get reward.

Player Limit

WoW:TaG currently has a maximum of 4 players. Runebound, depending on the number of expansions you own, can have 1 to 20 players.

In both games, you can play solo.

Combat

Combat in Runebound is a three step process. You fight on three different fronts at one time and have followers who can assist you. In WoW:TaG you only have one combat roll. In this way, WoW:TaG is faster but not as nearly strategic as Runebound. In Runebound, you can buy gear to make weak points on your hero stronger . In WoW:TaG, all you can hope for is having the upper hand in the combat roll.

In both games, the idea is to roll higher than a specific number value.

Economics

What differs greatly between the two is the economic system. Runebound requires your hero to collect money and then travel to a city to purchase goods (armor, weapons, followers, etc.) This provides an unlimited number of possibilities to build your hero but also requires you (the player) to spend a lot of time just trying to collect cash and then traveling to cities to shop. WoW:TaG, in contrast, does not have an economic system. You gain your gear by defeating challenges or trading in a gold counter for a blind draw from the Auction House.

Exploration

In Runebound, you roll a number of dice that have geographic symbols. You match the symbols to spaces on the board. If you have a 1:1 match, you can move through it. This random movement creates a longer game and can be frustrating. In contrast, WoW:TaG rolls a single dice and your character moves that many space (or not); however, your character can only move through spaces that are equal to or less than your hero's level.

Game Components

Game components are equal across both games. Miniatures, cards, and bits are used, more or less, in the same way to represent the hero, the monsters, health, and other stats the player needs to keep track of.

Expansions

Runebound has many expansions and is a very mature game. In contrast, WoW:TaG is very new and only has the base game. An expansion is planned for WoW:TaG.

End Game

In Runebound, the end game is defined by the end game being played. Meaning, if you have different expansions, you can choose different end games and thus alter the game speed and player focus. You can also use the "Doom Clock" that starts the end game automatically after "X" rounds, forcing the game to end with a final conflict the players may or may not be ready for.

In WoW:TaG, the game ends when one or more players has 8 or more Valor points.

Player Interaction

Depending on the game type you are playing, Runebound will either require a lot of player interaction (either working as a group or against each other), or as neutral parties that can (if they choose) attack each other.

In WoW:TaG the only player interaction is the occasional character combat that is only necessary if you want to attempt to slightly weaken another character.

In Conclusion

Both games are very similar in many areas. The "feeling" of the game and the settings widely differ. Personally, I own and enjoy playing both games. I have found WoW:TaG to be a "light Runebound" in many respects and allows me to scratch my itch. However, there is not substitute for the level of play in Runebound.

I encourage you to try both, if at all possible. Both games are fun to play and worth while.