High quality musicians, good director, and nice programs year-round. Best ones are concerts that feature solo musicians. These might include a piano or violin concerto who showcase virtuosity while the DSO accompanies the soloist. Soloists are usually international stars, so it's an honor to attend one of their performances. Tickets can range from about $15 (sitting on the benches of choir) to $120 (box seats).
If the only classical music you've ever heard was on the radio or on tv, you really need to experience it live. Give it a chance for once. Classical music is really meant to be listened in a live setting because acoustics mean everything.