Raid Finder currently struggles from a bit of a double identity. On one side of things, we want it to be a fun, low-stress environment for people who want to be able to experience new raids, but would rather not devote the necessary time from their week that joining a full-time raid team can often require. Something that you can just jump into, see the new bosses, and be done with in, say, an hour or so, without too much concern.
At the same time, however, Raid Finder has been trying to fill in as a form of endgame progression for a lot of players. There's a very real difference between the casual player who just wants to see the new stuff, and the raider who can't commit to a regular team but still wants some meaningful progression. Prior to Patch 5.4, Raid Finder was all that player really had, so we wanted to provide them as much gameplay as we could while still considering the needs of the "sightseer". Not exactly an easy task, and although we've been happy with what we've accomplished so far, we feel we can do better.
That's where Flexible Mode came in. We've been absolutely thrilled to see how well Flex has worked out, not just technically but also in how much the community has embraced it. We originally designed the feature focused on smaller or less hardcore guilds who maybe couldn't guarantee a full roster every time they wanted to raid, but we've been pleased to see that it's also been working extremely well for pick up groups (particularly through third-party services such as OpenRaid or oQueue).
In Warlords of Draenor, we're implementing a completely new Group Finder, which will allow players to find or create cross-realm groups for pretty much anything, including raids. We think that sort of environment -- finding an organized group, with a leader, and strategy, and possibly even using voice chat -- will be much more appealing and enjoyable for the player who craves an endgame raiding experience but can't find a team that fits their schedule. Plus, having an easy-to-use in-game Group Finder will, we believe, make finding such a group just as easy as queueing for Raid Finder (or easier, if you often find yourself in Raid Finder groups you'd rather not be a part of).
That, in turn, will allow us to take another look at Raid Finder and how exactly it should be tuned. I don't have any specifics to share (we simply aren't at that point in development yet), but our hope is that we'll be able to better provide for both the "busy raider" and "sightseer" styles of gameplay as a result.
In summary: if you're looking for meaningful endgame raid experience, we'd like you to do Flex, and we want to make it easier for you to do that. If you'd just like to see new stuff (or even just take a laid-back approach to raiding), we'd like you to use Raid Finder, and we want to make sure that's fun for you.