Software products built to scale.

The hard parts of SaaS are the ones that are painful to add later. Multi-tenancy — keeping each customer's data properly isolated — belongs in the first architecture conversation, because introducing it after launch is difficult and getting it wrong once you have real customers is worse. Subscription billing is where a surprising number of otherwise solid products break: trials, upgrades, proration, failed payments and dunning are each an edge case that only shows up in production. Beyond that, growth exposes whichever performance assumption was made without thinking about it, since the query that was fine at a hundred accounts can be an outage at ten thousand.