That post length limit is what made Twitter interesting for me. This is being identified as a problem, however there's always Facebook and Google+ that don't have a limit. Twitter will not succeed by copying competitors.
> 2. Limit the number of tweets per day instead. 3. You can pay to remove the above limit.
People will never pay for removing such limits. They'll simply switch service once it gets annoying enough. This is the flaw in the logic of people that are anti ads as a business model. Basically people will not pay up unless there's scarcity and artificial scarcity doesn't really work.
Such a strategy would also be really unwise, because from my own observations, a majority of Twitter users are fairly silent, being content consumers rather than producers. So alienating those users that produce content for you is not the best strategy.