Resteems are very powerful for people who have a lot of quality followers.
When they resteem the post of a less powerful user, that presents the more powerful person's followers with the new person's content. It is an excellent way to gain followers for the new person.
The second best way to gain quality followers is to have your friends upvote a good reply to an existing user's trending or otherwise popular post.
What about the upvotes? Are there adverse effects to over-using them?
It uses your voting power. You only get so much, and it slowly recovers over time. The steemd.com and other sites will tell you what your power is. The more steem power your account has, the more power your vote has.