We implement rate limits on users as a preventative anti-spam measure. We've thought long and hard as to try and produce limits which will be effective in preventing inauthentic site behavior, while also not adversely punishing genuine users. Currently the rate limits are as follows:
Subscribe: 150 in 5 minutes | 10 per minute | 200 per hour | 400 per day
Quote/Remind: 150 in 5 minutes | 10 per minute | 500 per day
Up Vote: 150 in 5 minutes | 10 per minute | 1000 per day
Down Vote: 150 in 5 minutes | 10 per minute | 100 per day
Comment: 150 in 5 minutes | 10 per minute | 500 per day
Boost: 5000 total views accumulated across all boosted content per day