No seating plan!?

Sounds like a great idea! Nothing worse than sitting next to people you don't know and having to make small talk. At least this way people can sit next to who they want to sit next to.

Sounds like it should be okay, I would recommend having reserved seats for the bridal party or anyone who has to sit somewhere in particular, and having ushers ensure people are filling seats from the end so that you don't end up with odd gaps where groups have to spilt up to fill them (kinda like when people get in a bus in a group and there aren't seats together)

We had no seating plan. Sweetheart table for us, family tables for my family and his family, then everyone else sorted themselves out. We deliberately had about 20 more seats than invited guests so they had plenty of choice. Ended up with one empty table and the guests all seated themselves with no fuss.