Clear diction is one of the most important goals you can set for your choir. We've all heard a choir that sounded nice musically, but didn't clearly convey the text. This is not enjoyable for your audience and will not draw people to your performances. Encourage your choir to over-emphasize the consonants. If they don't feel like they're overdoing it, they are probably not being clear enough. Give your choir examples of what you want words to sound like. If you are preparing a piece in another language, research to be sure you are instructing your choir in proper diction for that language.
A well-blended choir is always a pleasure to hear. You might need some rehearsal time of straight-toned (no vibrato) singing in order to focus on blend. A well-blended choir will be balanced in parts, volume, tone and vowel placement. Make sure that you have a good mixture of strong singers on each part. If you have one member who sings above everyone else, speak to him in private rather than embarrassing him in front of the entire group. Encourage each singer to blend well with the person next to him both in volume and tone. Vowels are perhaps the most difficult sounds to blend. If some of your singers are placing vowels in the front of their face, while others are placing them in the back of their throat, you are not going to achieve good blend. Demonstrate for your choir where they should be placing their vowels.
Make it your goal to have enjoyable rehearsals. A lot of this depends on you as the director. Maintain a focused yet relaxed atmosphere. Everyone likes to be challenged by an inspiring leader. But most people don't enjoy being driven by someone who is too intense and too focused on perfection to the point of taking the pleasure out of something you love. Depend less on talk and more on demonstration. Don't get too technical in your instructions. Instead, think of creative ways to get your choir to produce the sound you want. Mental images work very well for this and make rehearsals more fun.
As you plan your concert repertoire, choose pieces that will be accessible to your choir and enjoyable for both the choir and audience. Your choir will enjoy singing something that is a little bit of a challenge, but not too overwhelming. If your choir has fun with the music, the audience is more likely to enjoy it. Consider choosing a theme, then select repertoire around that theme. Don't be afraid to try something a little out of the ordinary. Try a more modern piece with unusual techniques if you feel your choir could master it.