First rounders are big time gambles. They net a bust more often than they do a superstar. Basically by giving up a pick for Felton you're trading a door that could have coal, silver, or gold behind it for a door that's guaranteed to have silver behind it. Yes you're giving up your shot at gold but you're eliminating the possibility of coming up with coal, too.
One could also argue that silver (or perhaps bronze, in Felton's case) isn't going to get us a championship. We believe we've hit gold in Tyreke, we hope we've hit gold in Cousins, and one more hit (and a few years of gelling) should get us into the discussion, while silver is unlikely to. And some of us have a good deal of faith in Petrie dowsing out that gold yet again, and high first rounders don't bust out as often as lower ones.
As a further note of the value of first-round picks, see the "Ted Stepien" rule - where the NBA instituted a rule that a team could not trade away consecutive future first-rounders because they recognized Stepien was ruining the Cavs by trading draft picks left and right for silver/bronze vets. This doesn't mean you should never trade away a pick. it just indicates that the expected value of a pick is pretty high. In fact, it might be fun to look at top-5 picks that have been traded (rather than swapped for other high picks) recently to get a feel for the value.
2009: Washington traded #5 to Minnesota for Foye and Mike Miller
2007: Boston traded #5 to Seattle for Ray Allen
2004: Washington traded #5 and Stackhouse to Dallas for Jamison
2003: Memphis traded eventual #2 to Detroit for Otis Thorpe. The trade was executed in 1997, six years earlier, apparently without protection. This was probably not anticipated to be a #2 pick
2001: Clippers traded #2 to Chicago for Elton Brand
2001: Atlanta Hawks traded #3 to Vancouver for Shareef Abdur-Rahim
So that makes a total of 6 top-5 picks traded for established players in the last 10 years. One was traded 6 years beforehand in a move that lacked foresight. One netted a very disappointing return of Miller and Foye and was roundly abused at the time. The others were all for star-level players. Top-five picks are valuable, and we should treat ours as such. I personally don't believe that Felton has that kind of value.