I don't think WWE intentionally buries people just because they were in TNA. It is all about ratings. TNA Impact has comparable ratings to ECW (WWE's 3rd highest rated show), so how can WWE put guys who are not exposed to the WWE fans in main event feuds.
-Christian has not been buried, they put him in a program with the ECW champion (remember TNA and ECW have similar ratings), and ultimately gave him the belt on PPV, with a decent reign as champ. He was not seen by the majority of WWE audience in 3 years, and when he was in WWE before, he was a mid-card/upper mid-card worker, so why would he come in and beat HHH or Orton? The fans would crap all over it.
-R-Truth debuted on Smackdown, and was not able to get over enough to justify getting any sort of title shot or PPV feud.
-Chris Harris came into the company looking like crap, and got fired because he looked terrible, and couldn't get over.
-Gail Kim debuted beating up the Diva's Champ, that's a good debut, and she has only been back less then 6 months.
Even if WWE were to get their hands on one of TNA's young talents (like AJ Styles, Samoa Joe, or Christopher Daniels), they should still be put in mid-card (or on ECW), because they are not known to the majority of WWE fans, and as such, fans would not believe they belong in the Main Event.
Back in the day, WWE did hire a lot of ex-WCW talent (Austin, Foley, Jericho, Guerrero, etc). However, none of these guys debuted in WWE winning the WWE title. It took Austin 2+ years before he got over as "Stone Cold," Foley took about 4 years before he became a main eventer, Jericho took 2 years in the mid-card before becoming the First Undisputed Champion in WWE History, and Eddie took 5 years. WWE needs to get it's fans to believe in their wrestlers before pushing them to the Moon.
The one guy I want to see WWE pick up is Samoa Joe, he has a different look, and is an awesome in-ring talent. TNA just put him in the Main Event Mafia (AKA WWE south), which makes no sense. I would also like them to bring in Christopher Daniels, who TNA for some reason decided to call "CURRY MAN" for a year, before bringing him back and putting him in a feud with SHANE DOUGLAS?! WWE should try to snatch both of them up, give him 4 months on FCW, in order to learn the WWE ring, then debut them on ECW (AS SAMOA JOE AND CHRISTOPHER DANIELS).