7 Wonders is the rare game that scales beautifully from three to seven players without slowing down. Everyone drafts simultaneously, so adding players doesn't add turn time — only complexity.
Three win conditions interact in shifting ways: civilian (blue cards), scientific (green), and military (red). Hard-committing to one funnels you straight into your neighbours' counter-strategy; balancing across two is usually the safer play.
The Cities and Leaders expansions are good. The Pantheon expansion is widely considered fluff. Stay close to the base box for the first 10-20 plays.