and all begin with the same word: Choice
1. Choice for users: Bing is the 1st real competitor to Google. For 10 years Google has dominated the search scene with its fabulous search engine. But I felt the last 5 years, the search engine has stangnated. It didn't add any new interesting functionality, it didn't improve its image search and its video search is quite basic. Now the choice exists. I now prefer Bing's Video search (remarkably better) and have also started checking search results on both Google and Bing.
2. Choice for advertisers: Rates on Google for many spots were getting quite absurd (thus the superhigh profits earned by Google!). Because Bing is bound to get good market share over the years, advertisers will have a choice, which will drive down ad rates.
3. Choice for websites: People tend to forget that search engines are nothing but a collection of data from all websites. The problem with Google has been is that websites, such as my Flora2000.com have had to remain pretty stagnant from a content perspective because the search engine with 70% of the market, did not adopt any new ranking methods for the last 5 years! Now with Bing, hopefully things will change and each Search Engine is sure to outdo the others and release new features. Websites can now choose to release features keeping in mind that it will show up on one search engine but may not on the other, which is ok, if both have decent market share.
I really admire Microsoft for coming from behind once again with a strong product. I'd watchout if I were Google, because MS beat the leader in the Spreadsheet (Lotus), the wordprocessor (Wordperfect), the Browser (Netscape), the Gaming Machine (Nintendo, Sega), to dominate the space.