After Season 3 ends, League of Legends will no longer 'clamp' League Point gains for players below Diamond tier, so that everyone advances through the ranks a little bit faster and combat a trend of players reaching a tier's fifth division, burn out of low LP returns and become "increasingly toxic".
Additionally, players can now be demoted through the tiers, where previously you were immune from dropping. This, according to a post made by Riot's Socrates on Reddit, is also to help combat the trend of players ending up in a tier where they're no longer competitive for one reason or another and being unable to advance through the tier. This apparently leads to toxic and dissatisfied players.
Now after 10 invulnerable games you'll be at risk of dropping a tier if you're not careful, although Riot has pointed out they'll take care to warn you of any impending demotion so that you can pick yourself up and watch some tutorials on Youtube.
Lastly, Riot are also hoping these changes will allow more people to reach Diamond tier, where the developer says there isn't enough of a player base to efficiently find skill appropriate matches. In particular there's a problem with the number of players at 100 League Points in Diamond facing off against those with 0, who can't cope against the best League has to offer.