At 100 reputation, you can only edit community wiki posts. Posts are made community wiki in exceptional situations where the wider-spread editing is desired. That's why it has the lower reputation requirements.
In the normal use case, editing other people's posts requires 1,000 reputation.
From the faq — What is reputation?
15 Vote up
15 Flag for moderator attention
50 Leave comments†
100 Edit community wiki posts
125 Vote down (costs 1 rep on answers)
150 Create new tags
200 Reduced advertising
200 Retag questions
250 Vote to close, reopen, or migrate your questions
500 Vote to close, reopen, or migrate any questions
750 Show total up and down vote counts
1000 Edit other people’s posts, vote to approve or reject suggested edits
1500 Vote to approve or reject suggested tag wiki edits
2000 Vote to delete closed questions, access to moderation tools
3500 Protect questions so only registered users with 10 rep on this site can answer
4000 Vote to delete negatively voted answers and stronger question deletion votes
Note — These are the reduced reputation requirements that are in effect during the public beta.