The clarity of your voice and a comfortable volume may be the most essential parts of great audio. If your voice over recording is fuzzy or muddy sounding, it will be difficult for people to understand. Audiences likely will be distracted and unable to absorb the information or may simply move on. Either way, they miss your message and you miss an opportunity to share what your knowledge.
The quality of your voice over depends almost entirely o the skill of the voice actor you use. A good voice actor will have perfect enunciation (ie, be very easy to understand), speak at an ideal pace (not too fast and not too slow), and have a pleasing voice. They should also be good at acting, and able to convey the feeling or emotions required by the script, using vocal variance and tone to keep the words interesting while also embodying the desired mood.

In late 2009, my wife Simone received an email from Prague in the Czech Republic. The sender, Mirek Jirku, was heading up a project to record voice messages for the foreign‑language versions of the latest Avast! anti‑virus software. Mirek sent us eight messages to be recorded, specified the fee and deadline, and requested a slow, friendly read. He asked us to send separate stereo MP3 files (at 160kbps or higher quality) for each message, specified that we'd have to sign a non‑disclosure agreement before recording, that we'd need to invoice afterwards, and that he'd pay via a wire transfer instead of PayPal (which is often used for international payments).
As I've mentioned, most of our clients prefer to do post‑production work themselves, and request the raw (but edited) recordings. Sometimes, though, as in the Avast! job, the client will want the files ready to go, which might mean using slight EQ and compression. The noise floor on my recordings generally hovers around ‑54dB, but by using EQ to roll off frequencies below 80Hz and above 12kHz, I am usually able to lower the noise floor to around ‑57dB without resorting to noise reduction. Using EQ to reduce low‑end rumble and high‑frequency noise will make your recording sound better, and will be particularly beneficial if you'll be adding compression later, as the compressor's make-up gain won't be raising the level of that noise. Used moderately, compression can usefully reduce dynamic range, and thus improve intelligibility of the words, while still leaving you with a natural‑sounding result. Used excessively, it will suck the life out of a recording.
As editor, you need to turn the raw recording into the final version of the file you'll be sending to your client. Editing during a recording session can be problematic, and particularly so if you're recording in a domestic environment. How can you expect to hear the drone of the dishwasher ruining your recording if the dishwasher is still running as you edit? I'll look in more detail at setting up a voiceover recording studio next time, but for now, if recording in a separate room or isolated booth isn't an option, at least try to edit the file the following morning (or maybe at night) when there's less noise around — because hearing your recording with a fresh ear and in a quieter environment will help you to make better editing decisions, and will reveal noises that slipped your attention earlier.