1. Good volume/liquidity - the page should be responsive too so orders trigger on time and can be cancelled quickly
2. A good reputation - reasonable care is put into how users funds are stored (I. E its not all in a hot wallet).
3. Fast and cheap withdrawals with just 2fa (password plus authenticator/sms)
4. Honest fee descriptions for leverage - how much you pay the market for your position every day and reasonable fees
5. If it has a good api and external charting software (like bybit and bitmex use trading view) its a lot faster and a lot more responsive that way. It makes it easier to cancel a trade if the site gets ddosed if they stillnallow trusted premium bots through... Or bots can keep pinging a request.
6. Compatibility with other platforms, can I access it on my mobile or do I have to be near a desktop pc?