Style of the meetup. Some meetups are purely “social” events, so often a restaurant or pub is an acceptable venue. However, many meetups like to have talks and networking without background noise, which means you'll need to find more of a dedicated venue.
Location & ease of access. Is the venue easy to get to by train/bus/tram/walking/car? What are the car parking facilities like?
Cost. Many venues will charge, so unless you can get a place for free, you'll need to factor in this cost, which means you may need to look into
sponsorship.
A/V Equipment. If you're meeting at dedicated meeting or conference venues, they will often be able to provide equipment such as projectors, screens and microphones (if required). Otherwise, you'll need to make sure you can borrow such equipment. If you don't have a screen, a plain white/cream wall will often be fine, so don't be afraid to do the best with what you have.
Facilities. Tea, coffee and water are important, but not every venue has the ability to provide these. Make sure your attendees know if they need to bring their own drinks.
Alcohol. If you would like to allow alcohol, you should check with your venue and/or local authority for what the regulations are. These rules will vary across the world, so it's best to check locally and that your venue are happy.
Accessibility. Although generally it's the venue's responsibility to make sure they are accessible, it might be worth checking to ensure your meetups are accessible to all, for example that there are ramps for wheelchair users.