Just notice that after simple site validation, my website was featured on google first page with my first keyword. Before this, my website had been indexed by google with the same keyword and didn't surfaced on the front page. Was this because i validated the site or what is it?
Validation can only ensure good crawlability and in turn quick indexing, so i don't think as you mentioned...may be, it is your sites quality content or few credible backlinks that drove Google to surface it up! (BTW congrats!!!)
please check again after one week in google. automatically it may change your site place in google search
Are you talking about google webmaster tool validation? I don't see that contributing to a site ranking first (unless we are talking about site's name). It will just help you find problems with crawling.
Other people have seen increases in ranking after validating. It may have just been that some issue with your previous code was interfering with the crawlers.
Validating your site for Google Webmaster Tools does not affect your ranking. Being logged in WMT while searching at Google may slightly vary the SERPs you see because of personalization... But you won't rank better because of validation. If your site was only recently indexed (last few days or weeks) it's probably the Google honeymoon where they show your page occasion blended in with the normal results on page 1 for a few days, occasionally a few weeks. They artificially bump you up so you get shown on page 1. Many theories why... Some say to get info needed for initial settings for certain ranking factors like click-thru-rate, bounce-back rates, etc. which can only be obtained if people see your listing. Others say the logic is built into the algorithm for newly indexed pages/sites to get late breaking news and new fads to the top of the SERPS... who knows. But after the honeymoon you fall way back into the SERPs where you SHOULD have been ranking all along. When people see this they often freak and think they are penalized... when in reality they never really "ranked" on page 1 to begin with.
Validation has no effect on rankings (unless the page doesn't show up). It could have been that the page was reindexed and move up.
Better the code, easier for bot to craw - true but don't believe that SERP position depends so much by this.
I validated the site with the google webmasters tool. before validation, the site had 214 errors and now it has just two.
Your most likely experience the Google dance, it takes months for a website to stabilize unless your targeting a very low competitive keyword.