Not only should the web host be reliable and fast, it should guarantee its uptime (the time when it is functional). Look for a minimum uptime of 99%. In fact, even 99% is actually too low - it really should be 99.5% or higher. The host should provide some sort of refund (eg prorated refund or discount) if it falls below that figure. Note though that guarantees are often hard to enforce from your end - the host usually requires all sorts of documentation. However, without that guarantee, the web host will have little incentive to ensure that its servers are running all the time.