Paladinsucks has the tip on this one: they link to this forum post by Darkhorse on Mannoroth that lays out the problem pretty well. Basically (and regular readers will know how bad I am at math), Blizzard has put a reduction to 60% on arena points earned from a 2v2 rating. The reason is because consider that it takes more skill to beat a 5man group than a 2man group. Here's the numbers crunch-- I'll be staying away from those, except to say that winning 10 arena games in 2v2 nets you only 335 arena points, while losing 10 5v5 games will still grant you 377 arena points. If gaining points is your goal, you're way better off messing around in 5v5 rather than clawing your way to the top through 2v2.
Now, not everyone is after points necessarily-- Blizzard obviously wants the 2v2 people to do it for the glory, not the points. And Drysc does point out that these numbers are only after one week of winning or losing-- if you continue to win in 2v2, you'll get lots more points than losing in any bracket, while if you continue to lose in 5v5, your points will start to drop pretty precipitously. As for solutions, the current setup is "working as intended," but lots of players are asking for the points reduction on the 2v2 and 3v3 matches to be weakened-- 2v2 shouldn't be worth as much as 5v5, but it shouldn't be worth nothing. Whether or not Blizzard will go for that remains to be seen. Looks like I better find three more people and make a 5v5 team.