I would like to start using the magento cronjob to send out review reminders automatically. Cron jobs have never run before (since its installation two years ago) on this installation if I'm correct. I wonder what will happen. Can this result in problems?
It shouldn't be an issue, check the table
cron_schedule in the database. The uncompleted tasks that are in there will be executed so in case you want to make a fresh start just TRUNCATE that table.
After that you can set up either
cron.sh and it will rebuild the table with the current cron tasks.
I would recommend installing Aoe Scheduler if you want more insight in what cronjobs are running on your Magento installation.