You can push talent too quickly and we've seen plenty examples of that in the past decade or so. Truth is, no matter who you are, you should not be pushed to the main event or awarded with a World Title reign within your first year in the company. No matter how good or ready you or WWE believe you are, it is always better to get a more gradual push that drags out for a while. If you are willing to take a more gradual push to the top, it shows you have more loyalty to the company.
A true WWE superstar should start with some sort of debut. I prefer a debut of an unknown wrestler being signed to a show (such as Evan Bourne, Sheamus, etc.) or a new superstar debuting after weeks of vignettes (such as Finlay, Kofi Kingston, Carlito, etc.) I have never been a fan of superstars debuting by attacking a superstar... I find it too generic, and all of them have the same purpose: to make an impact. That happened with The Great Khali, Brock Lesnar, and Drew McIntyre last year.
Now, a fast push to the top isn't always a horrible idea, but with the WWE, it just shouldn't happen. It just never works for a long period of time. A quick push can likely lead to their release, a departure, a less-than-memorable WWE career, among other things. A push that comes too quickly will likely have more negative effects than positive.
The names you mention: Sheamus, Wade Barrett, The Miz, and Kofi Kingston. I will breakdown each of their careers and how they push have or will culminate.
1. Sheamus's push was definitely too fast, and with just 3 months on ECW before being moved to Raw, it helped further that thought. Winning the ECW Championship too fast was easier to recover than winning the WWE Championship too fast. However, the fact that losing the title didn't halt his push and he was given credit for "injuring Triple H", and is now a 2-time WWE Champion is great for him.
2. Wade Barrett... his push can't come too fast, which is the entire purpose of NXT. You are given 15 weeks (if you last that long) to get the pros and now the WWE Universe to be more impressed with your work. By the time you win the competition, you have been on TV for over 3 months. WWE obviously believe in him if they paired him up with Chris Jericho. So, his push likely won't come too fast because his NXT angle will only get stronger, and by the time he gets his championship match, he will have gained much more credibility.
3. The Miz was not given much of a push until he had been in the company for 2 years. He lost Tough Enough and then was signed months later. His 2 years on Smackdown from 2005-2007 didn't have much highlights except for his talk show and hosting the "Diva Search". Not until he was drafted to ECW, did he start showing his wrestling ability and a more serious side. Now, he is a 2-time United States and Tag Team Champion.
4. Kofi Kingston had a great time on ECW. His feud with Shelton Benjamin was one of my favorites in all 4 years of ECW. In his first match as a member of the Raw roster, he wins the Intercontinental Championship by defeating Chris Jericho. He then wins the World Tag Team Championship with C.M. Punk. Enters the Money in the Bank match at Wrestlemania 25, later wins the United States Championship, holding on to it for 4 months, after countless title defenses. He then is entered in a feud with Randy Orton, entered the MitB match again, drafted to Smackdown, and is now a 2-time Intercontinental Champion.
So, quick pushes aren't always a bad thing. It's just better to keep it more gradual and take it one step at a time. It will lead to a longer, more decorated career.