If you don't think MLB needs replay watch this video. The best angle is at the end.
http://www.youtube.com/watch?v=goZAKhjG15k
NHL actually got this one right. Have another Ump/Ref in the booth reviewing all non-ball/strike calls and let him radio down or on-field radio up for quick reviews. 30 seconds on tight calls might save 5 minute rants by the managers.