"If Microsoft loses the trademark battle, it will have a much more difficult time fending off imitators that promote their products with variations of the same name. Company officials say customers would be in danger of buying inferior products and that rivals would effectively get a free ride at Microsoft's expense."
Generic ("beer" or "cold beer") and purely descriptive terms ("computer repair") cannot be trademarked. Terms that are suggestive, arbitrary, or fanciful can be registered, with fanciful getting the most protection.
However, if an otherwise descriptive term has become associated in the public mind with the products of one competitor, then the term acquires what is called a "secondary meaning" and can thereby be registered as a protectable trademark.
Secondary meaning usually is the result of a massive and long-term marketing campaign (e.g., "Windows").
That is the general terrain for this area. Using these standards, common words that appear in a dictionary can wind up being protected as trademarks.
I think the issue with "tweet" is not yet decided. Time will tell on how this will come out.
