After 10 years heading up the world’s largest e-commerce site, eBay CEO Meg Whitman will leave at the end of March and John Donahoe will take over. That much is certain.

But Wall Street remains cautious if Donahoe, a longtime management consultant, can really rev up eBay’s sales engine.

During his first earnings call with analysts Wednesday, Donahoe promised big changes in the next several weeks to help revive eBay’s core business. He discussed his plan to offer fixed-priced goods and an improved customer rating and support system to lure buyers. Next week eBay (EBAY) will lower fees for sellers to list items while taking a bigger cut on sales. “EBay’s best days are ahead,” Donahoe said.

Some analysts aren’t so sure. Despite reporting fourth-quarter revenue that exceeded Wall Street’s expectation, eBay offered disappointing guidance for the first quarter and for 2008. EBay’s fourth quarter revenues rose 27% from the previous year’s record to $2.18 billion. The San Jose-based company said its outlook for 2008 was between $8.5 to $8.7 billion and just $2.0-$2.05 billion for the first quarter. Shares of eBay fell 5 percent in after-hours trading Wednesday.

Some suspect that eBay is low-balling estimates to give the new CEO the best odds to look good. “That was a huge step down, and quite a bit lower than we were expecting,” says Jeffrey Lindsay, a senior analyst with Bernstein. “Maybe eBay didn’t want to set the bar too high for Donahoe, but that could backfire. After what happened with Apple, investors are extremely sensitive with guidance.”

Whitman grew eBay from a small auction site to an online giant with revenues of $7.7 billion in revenue last year. After a decade she decided it was time for a change. “It’s important that new perspective and new eyes come into this company,” Whitman said on the earnings conference call. “It’s difficult to stay fresh for this long. I felt it was the right time to turn over the reins.”

Donahoe’s methodical style is a drastic turn from Whitman, who was known as a big risk taker. Whitman often led by instinct, resulting in the acquisition of PayPal, which raked in $563 million for the fourth quarter, making up a quarter of eBay’s profits. But that risk-taking also led her to buy Skype in 2005. The company conceded it overpaid for the Internet calling service when it took a $1.4 billion charge in the third quarter of 2007.

Donahoe, who spent 18 years at consulting firm Bain, has a much more systematic approach. Several times during the call, he stressed that he would test new models by collecting data points first and would tweak models if they did not immediately succeed. Whitman hired Donahoe three years ago to run eBay Marketplaces, which makes up 70 percent of the company’s revenues. The division’s revenue for the fourth quarter rose to $1.5 billion, a 21% growth from the previous year’s period.

“He seems to be addressing many of the key issues, which is a nice change,” says Derek Brown, an analyst at Cantor Fitzgerald.  “He seems to be talking about making more aggressive changes. Whether he will be pulling the right levers at the right time at the right degrees, I don’t know. That’s the $40 billion question.”