While the dress and overall look is an important issue, the most important of all - and the reason why I'm here - is the music.
A good wedding piper is not a piping-champion. He's not there to perform a match to win a prize. He's at your wedding to perform the perfect soundtrack to it. Obviously he must have a deep knowlegde of the piping technique, a true skill to play slow marches in a cadenced and steady way, as well as quick marches with a brilliant accent. Furthermore one of the most important feature he must take care is about the perfect tuning of his instrument. Too many times I've heard a nice-to-see piper, well dressed, smart and tall, giving a poor performance because of a poorly tuned bagpipe or a poor fingering technique. Piping is a matter of continuous end exhausting practice to keep a good level, flawless fingering and rythm. In the last forty years I've been playing the Highland Bagpipe I've learned a lesson: there are no tricks and tips to improve your skill on the bagpipe but the rule of the three "P": practice, practice, practice!
That said, you should beware of poorly performing pipers. They can transform your wedding day into a laughing experience of a poor man struggling on his out-of-tune pipe sounding like a bag full of cats! Moreover if he hasn't the proper experience to play for long times without losing his concentration will be a true disaster. Always pretend to have a previous listening of himself playing the instrument. If he has video recordings it would be great. Listen to his style and tuning and choose only the ones who can afford a very long day of work and have a strong and long experience on playing in weddings. A young, even if talented, piper can strike yourself in the first hour with tricky flourishings, but if he hasn't the necessary strenght and experience his sound can become quickly erratic. And, as long a good photographer has always a backup camera in case of failures, a good experienced piper always works with a second chanter in his case, a handful of good spare reeds, an electronic tuner to tune his pipe amongst the shouting crowd typical of a wedding party. It's definitely a job he's payed for and he must face it like a pro.
If you have any special requests regarding a tune you love, tell him in advance, during the first chats. Not every tune fits the rigid and nine-noted scale of a Highland Bagpipe. Don't ask him please to play the last pop tune or some classical music that needs a group or an orchestra. The bagpipe has its own repertoire made of airs, slow tunes, hymns, marches, dances not to say about the Piobaireachd, the classical gaelic music written for the bagpipe, a solemn and hieratic music very old and considered THE music for the Highland Bagpipe.
Nowadays a skilled piper can have a reperotire of dozens of tunes. Let him choose the best tunes for your wedding. Typically a slow hymn like "Highland Cathedral" is the classical tune for the bride's entrance at wedding, while the ubiquitous "Scotland the Brave" is the classic for the way-out from the wedding venue (church, townhall..). During the ceremony, after the ring exchange, it is also a classic to play the lovely and well known tune "Amazing Grace" and it's not uncommon to see guests start to sing it with a terrific effect!
In the last years I have recorded in studio some famous tunes. have a look here at my YouTube channel
to see and listen to me playing the bagpipes.