How are the circuits and design considerations for opamps and comparators different, at an integrated transistor level?
The main difference is that opamps need to be stable when in a negative feedback configuration. This means that they need to be compensated. Compensation slows your slew rate.

For comparators stability is not such a big problem. All you need to ensure is that it saturates and doesn't oscillate.

Comparators also have two output states. This means that gates can be used improving the speed but they introduce parasitics. See this