I went through this application process last year and thankfully matched. From what it seem like, the programs are sending out invitations a bit later than last year. Having gone through the stress of waiting for interviews and checking my email 10+ times a day, I can say that most programs do not send out interviews until the middle of August. I still got interviews in late September and even November. The 10 or so programs that have sent out interviews so far are more of an exception then the rule. As for IMG and DO applicants, don't be disappointed even if you haven't gotten an interview from a place that has already sent out invitations. Programs send the first round of invites early and then more often then not send out the second round. Also, when the top applicants have gathered enough interviews, they will commonly cancel some of the ones that conflict with the invitations from other programs they want to go to. In that case "which happens all the times toward the second half of the interview process" the programs will offer up those spots to the rest of the applicant pool. I personally have gotten a lot more interviews starting the 3rd week of August - through the end of September (and even a couple in November).
As far as what the programs consider the most important things. From my experience, the letters of recommendation were most important. Yes research is important for the top 15 - 20 programs in the country, but for most other people as long as there is something in the research section - you will be okay. The programs do not expect you to have done a lot of research as they know that its tough during residency. If they see you had a poster presentation and a couple of abstracts published they will be happy.
Getting back to the letters of recommendations - a lot of Cards attendings know each other as long as they are within a similar geographic area. Because of that, a good letter from someone they know or may know of is a lot more important then anything else in your application. An interview where your interviewer recognizes your letter writer (as long as the letter is good) will definitely put you at the top of the list compared to others. If this interview is then followed up by a phone call from the person who wrote that letter to your interviewer -- you are in a very good position.
Every program has its own screening process since they cant look through every single application. These are usually related to Univ vs. community program, Visa situation, AMG vs. IMG vs. DO and sometimes Step scores. The top programs are probably more likely to screen out most of the applicants except for those from other top programs. However, many programs are happy to interview a much broader applicant pool.
Remember, its still really early - don't give up - good luck everyone!