From
http://www.nintendo.com/whatsnew/detail/vyWpoM6CBIe6FjW8NIY7bvzOrgBURhzw
As of May 20, 2014, certain online functionality offered through Nintendo Wi-Fi Connection will no longer be accessible. The discontinued services include online play, matchmaking and leaderboards for many Wii, Nintendo DS and Nintendo DSi games. For a comprehensive list of games and services that are affected, please check
http://support.nintendo.com/servicesupdate. Users can still play the games in offline mode, which continues to offer a rich game-play experience.
Online play for Wii U and Nintendo 3DS games will be unaffected (aside from the Wii mode on Wii U and Nintendo DS games on Nintendo 3DS family systems). Other online functionality, such as access to the Wii Shop Channel, the Nintendo DSi Shop and video-on-demand services, are also not affected at this time.
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This doesn't really affect me but there's a sizeable list of games it affects (though Monster Hunter Tri wasn't on the list) that are all listed in the link at the beginning.