One thing that bothers me is that people seem to be so hesitant to include any modern works. I know so many pianists that play some greatly underappreciated or underplayed pieces but are afraid to play it because they "don't know if the audience will like it". It seems almost like the contrary though. Obviously just cavalierly tossing in some hyper-atonal work by Finnissy or something like that is not the way to go, but the works of Rzewski, Ligeti, Roslavets, Mosolov, Carter, Messiaen, Berg, Boulez (specifically Sonata No. 1 and Douze Notations), Vine etc. are no "worse" than Mozart K. 576, and that in contradiction to what these pianists think the audience quite often goes wild over them! I'm just sick of seeing a program like say...
Haydn Sonata
Schumann Kinderszenen
various chopin etudes
Liszt consolations
appassionata
rach etudes
chances are, you're not going to give a performance of any of these pieces that anyone will go "wow! that is the best interpretation ever!" because everyone's heard all of them six thousand times unless you're Bolet, but in the modern works there is often no sort of interpretational rules that some music critic elitist has decided is the correct way to play the piece, so you're often freer to take more liberty with your own personal interpretation, plus it adds variety, and gets the music out there.
I also see some programs where nearly every piece is in a minor key, if not literally every piece. That gets pretty boring. There needs to be diversity on the much more fundamental levels I think than say "oh I have two romantic pieces so I need two classical pieces". Keeping a diversity in the "color" or "emotion" of a piece I think is a lot more important when it comes to keeping the audience interested. A diversity in eras of music is only fundamentally useful if you're trying to prove you can play music from all of the selected eras IE a competition, which a lot of people seem to get mixed up with a concert program. For my own personal preference I often prefer concerts that have a theme, such as say something like:
Beethoven Sonata "Hammerklavier"
Debussy Etudes Book II
Boulez Deuxieme sonate pour piano (2me sonate is based on hammerklavier and the etudes)
or a program that focuses on one composer but includes pieces that contrast in various ways inbetween his/her works. Or one that does one thing in the first half of the concert and something completely different after the intermission. Although in doing these sort of things you have to be careful to not compromise the musical experience for the aesthetic of your programme booklet

In regards to length of a concert, I'm not very picky. If I'm buying a ticket just to go see you though I'm looking for at least 75 minutes. If it's a small festival concert then obviously it doesn't need to be nearly as long. I don't particularly have a problem with very long concerts. I know that Hamelin has performed some that were ferociously long, well over 4 hours if I am correct! While I would certainly not recommend or personally attempt something like that, I would not have any hesitation going to a two hour concert if the music was good and so was the performer.
On the subject of encores, I really hate it when the encore is just some totally random piece. I think it needs to somehow meld in with the rest of the program. For instance, if you had played a Liszt piece somewhere, then I'm sure it wouldn't be very tough to find a liszt piece to use as an encore. I always prefer something with some substance; frequently I will see that the pianist chooses a piece that is purely technical for their encore(s) which is entirely unnecessary considering there are plenty of pieces that have both substance and incredible technical feats. I would much prefer to see Mazeppa than Grande Galop Chromatique.
Anyway, randomly I think that Jonathan Powell does some of the best programs, and I urge everyone to check out his website and look through them

They're always very inventive and contrasting. Perhaps they will inspire those who might look at this thread for ideas.