I've noticed this problem lately. Last night, I saw that a large percentage of my front-page was from 1 subreddit. I'm also subscribed to over 200 subreddits. Some of these subreddits never hit my frontpage, and I tend to visit only 10-20 subreddits. This results in the majority of my subreddit subscriptions being useless.

I think there could be some interesting UI solutions to this problem. If more people treated the Reddit API like the Twitter API, there could be applications that aren't necessarily supposed to replace the traditional Reddit browsing experience, but to make whole new experiences (e.g. Flipboard).