Yeah, I was frustrated with Arena. I would gain ground when I did my 5, but would lose most of my progress throughout the day, even though my offense and defense were the same. I hit mid 1200's briefly but mostly finished between 1500-2000. The raids were getting to be tough -- I was level 51 but struggled halfway through the 40 raids because my lower characters were so weak. For me to be better at these features, I had to step it up quite a bit. And I was losing ground in the Campaigns. I hadn't completed either the Nexus or Villain campaigns, and got my butt kicked with the first cosmic battles.
For M:AA, I played from the beginning to the bitter end, achieving pretty much everything I wanted to. But I couldn't put myself through that again. I have a very competitive nature, and all or nothing. I had to bow out when I did.