The list of mods that are arguably more popular and influential than the base game starts with Counter-Strike and ends with Defense of the Ancients. Valve hired the Counter-Strike crew. Part of the Defense of the Ancients brain trust started up Riot Games. Existing players will welcome League of Legend's improved interface, graphics, and variety. However, despite a fair amount of effort on Riot's part, it's nearly as impenetrable to newbies as the notoriously unfriendly original.
The basic schtick is to use a single champion unit, which starts at level one with just a handful of gold, to turn the tide of battle between two NPC armies that continually spawn at opposite corners of the map and run headlong into each other. You're constantly struggling to stay ahead of the power curve as experience (and more importantly, gold) improves every champion on the field over time. Purchased items have a massive effect on champion power; it's not uncommon for a "carry" player (someone fed gold and protected by his team with the goal of becoming overpowered in the endgame) to consistently two- or three-shot champions toward the end of a round. There's a lot of room for strategy in how you get to that goal, and riding that power curve is great fun.
Early game jousting lasts for about ten minutes. If nobody has done anything stupid enough to get themselves killed and thereby pushed an enemy ahead of the power curve, the next 15 or so minutes almost invariably determine the match as the kills and bounties pile up. Then you get to act out a script for the next half hour while one side slowly grinds down the other. Comebacks can and do happen, but rarely enough that the endgame slog is painful for the losing team to play out.
The rich-get-richer effect of the gold/power curve and brutal punishment of early mistakes mean that having a newbie on your team is worse than having that slot be empty. This has the predictable effect of making the community – already spastic due to the game's PvP nature – extremely hostile to newcomers. League of Legend's matchmaking system helps a bit in this regard, but not enough.
It's too bad that Riot Games didn't take more chances with League of Legends, because the basic premise is good enough to have made Defense of the Ancients a subgenre unto itself. The window dressing – the persistent upgrade metagame, much-improved interface, and solid matchmaking – is a significant step forward. The core gameplay is identical, but shares the same problems. As flawed as it is, at least Demigod presented some new ideas. League of Legends is for Defense of the Ancients veterans and few others.