One of the methodologies behind logo design is creating a shape. The reason to create a shape, is so that your logo becomes instantly recognisable without even having to read it closely. The coke "ribbon" text/shape is one of those that you barely need to read before you know what it is.

Kerning is the term used to describe the white space between each letter, when there is more space between letters, the word as a whole tends to lose its "shape". Most logo designers reduce the kerning of text to keep the eye moving around the shape, and almost every logo you care to name (bar a few) will keep the letters very close together. Here's a few very recognisable logos to show you what I mean - notice how closely kerned the letters are:

The other thing you may notice, is the font choice. Most use a plain sans serif font. That is, there are no "ends" on the letters such as a font like Times New Roman... On that note, several fonts are banned from logo design, they include such fonts as MS Comis Sans, Curlz MT and Times New Roman - just don't even think about using them, as they look extremely unprofessional.

Lastly, different fonts in the same logo are frowned upon in general. Unless it's a something very artistic that requires it, you really should try to stick to using the same typeface, even if you have 2 or more words combined. Try experimenting with different colours, shapes, font weights or sizes if you think you need to draw attention to a word or letter. Don't be afraid to "tweak" a letter to make your logo that much more unique, the microsoft and dell logos both have a single letter tweaked to make it stand out. Good luck!