Good execution means low slippage on market orders, no requotes, and consistent fills even during volatile conditions. It is the hardest thing to evaluate from a broker's marketing material — you have to either trade live or read independent execution reports.
The main factors that drive execution quality are: the liquidity providers the broker connects to, the latency between the client and the matching engine, and whether there is a dealing desk intervening. Brokers that publish monthly execution statistics (percentage filled at requested price, median slippage, etc.) are usually the ones worth trusting.