I don't think some of the explanations of 'equal temperament' have it quite right. There are many versions of equal temperament, but the most commonly used in Western music is the 12 tone version.

The system is intended for fixed-pitch instruments (keyed & fretted mainly) and is designed to make a scale played in any key sound the same. This means that all major triads (3-note chords) sound the same, and all of them sound equally out of tune.

The alternative 'just intonation' for a fixed-pitch instrument makes chords in one key sound perfect, and other keys out by varying amounts.

I could explain the theory, but I'd be here typing until midnight. The essence of the explanation is given clearly in two Wikipedia entries. Frequencies in 12-tone equal temperament are compared with just intonation in a table under the heading Comparison to Just Intonation in this article, and there are 3 sound files which illustrate the differences here.

The idea that some keys sound "brighter" than others is widespread - I prefer most hymn tunes in Ab rather than A, for example. However, a number of my musical friends do not agree, and I think this effect is a very personal thing. The first Wiki article includes: (Correspondingly, there is a great deal of variety in the particular opinions of composers about the moods and colors of particular keys.)

Finally, I wonder how many pianos are actually tuned exactly to equal temperament. Unless an electronic tuner is used for each of the 12 notes (only a recent possibility) the tuner is trying to "detune" notes by a very small amount from the perfect 4ths and 5ths that we all hear fairly easily. Just look at some of the differences in that table.

I know I always tune the top E string of my guitar up a fraction if I'm playing in C or G - it just sounds better to me. The rest of you don't have to listen!

Phil