Running relays and swimming relays are different in lots of ways, some of which have already been described.
Here are some others:

In track you pass a baton during the race and moving that baton around the track in the the fastest and smoothest way is what you strive to do.
In swimming no baton is used.
In track there is also a 10 meter passing zone in every lane within which each baton pass must be completed.
Officials watch to see that every baton pass is totally completed within that zone, each of which is ten meters in length.
You can plan for your fastest runner to run a significant number of meters farther on his leg by having him or her receive the baton early and/or in the passing zone.
You might also plan for your slowest runner to run the shortest distance possible, by having him receive the baton late in the that leg's initial passing zone and then handing it off to the next runner early in the leg's second passing zone.
Thus your slowest runner would carry the baton for the shortest distance.
For example, if you wanted, you could have your first runner carry the baton 90 meters, your second go 120 meters, your third go 80 meters and your anchor go 110 meters, as long as all the handoffs get made just within the passing zones.
In comparison, you can't manipulate the individual distance that each relay swimmer swims.

Some runners are better coming out of a starting block than others and, of course, you want a good block starter on the first leg.
Also, you might need your second fastest runner to pass the baton to your very fastest runner because the second fastest might need that speed to more successfully match up to the fastest in order to get the baton to him without making the fastest runner delay in almost reaching his top speed because he's waiting on the baton pass from his slower team mate.
This match-up consideration is not there in swimming relays.
Making a smooth, efficient baton pass is every bit as as important as running fast.
In a pool, every swimmer must swim a fixed distance and there are no starting blocks, so the above factors don't come into play for relays in the pool.
Swimmers do need good starts and turns, they are very important, but every competitor has an equal opportunity to start and turn on his leg.
And those legs are always a fixed distance, because, in a 400 meter swimming relay for example, each swimmer goes exactly 100 meters.
In a 400 meter track relay, you could have your fastest runner run almost 110 meters, if you have him run the last leg and receive the baton early in the passing zone from a 3rd leg runner who is fast enough to pass the baton to him just as the anchorman was about to reach his top speed.
Moving the baton from the start to the finish in the smoothest, fastest way is the goal.
In swimming relays, those races in which each swimmer swims a different stroke have all the backstrokers, butterfly strokers, breast strokers and American crawl strokes swimming in a stroke specific order that each team has to follow.
You can't change the stroke order or the total distance each swimmer has to go.
In track, you can choose your running order and also significantly manipulate the distance each runner carries the baton for your team.

In track, you could start with a good block starter who runs a good curve handing off to a slow second runner late in the first passing zone.
The second runner could hand off to your second fastest runner early in the next passing zone.
That third runner could use his top speed to efficiently get the baton to your very fastest runner, just before that runner reached top speed to anchor the race.

Or a good block starter could pass the baton after 90 meters early in the first passing zone to your fastest runner, who could then carry the baton about 120 meters to nearly the end of the next passing zone before handing off to your third and slowest runner early in the next passing zone.
That runner could carry the baton only about 80 meters on the third leg before handing off to the final runner, who you hope has enough speed as your second fastest to anchor your team home over the last 110 meters.

All this decided after considering who can run curves at their top speed and who are the best at handing off and/or receiving the baton, since your first and last runners are involved in only one baton pass rather than two.

Or, if your fastest runner is also your very, very best block starter, he could go first and carry the baton almost 110 meters and then ......

Such running relay manipulation and leg placement might be called either good coaching or over thinking, depending on how successfully your athletes carry out your plan.

Gig 'em, FAST FRED '65.
Before the world wide web, village idiots usually stayed in their own village.
[This message has been edited by FAST FRED (edited 8/18/2008 12:22a).]