Here's my take on the front-runners so far:
President Trump: Various Republicans are planning to challenge him in the primaries. He will smash them just as he did the other Republicans in 2016.
Joe Biden: It's common for a Vice President to run for the presidency but it's rare for them to win. George H. W. Bush is the only one to have done so (plus Nixon on his second try running against Vice President Humphrey). Biden is following Hillary Clinton's lead and running as the presumptive candidate. Like her, he teased his candidacy for months then announced that he'd be announcing before finally announcing with a video. Biden is already looking past the primaries. His announcement video's message was "Orange man bad, vote for me". Hillary's 2016 campaign showed that you have to do more than attack Trump if you want to win the White House. Also, like Hillary, Biden carries a lot of baggage. He was first elected in 1972. He's always supported mainstream Democrat positions but the party has drifted so far to the left that votes for tougher sentencing and the Defense of Marriage Act in the 90s, both of which were overwhelmingly supported by the Democratic Party, are now negatives. Biden has a reputation as a gaffe machine and his campaign is following his lead. On his announcement day he received criticism for misrepresenting what President Trump actually said about Charlottesville, using Charlottesville as a campaign prop, his treatment of Anita Hill in 1991, and releasing a picture with a big "N" on President Obama. Biden has name recognition and some reflected glory from Obama but little else. Binden's going to have difficulty securing the nomination but he's the Democrats' best chance at winning back the White House.
Bernie Sanders: In 2016 Bernie was the fresh face of the Democrats (and the oldest person running). He brought a far-left agenda which excited a lot of younger Democrats. He originally ran in an effort to move the party to the left. He was so successful that his positions are now accepted by early all the other candidates. That's going to cause problems for Bernie. In 2016 he pledged to run a positive campaign but once he realized that he might have a shot at winning his natural crankiness came out. He's going to have a huge advantage in fund-raising but he's ancient and crotchety and probably too far to the left to beat Trump.
Elizabeth Warren: Her time came and went six year ago. She seceded her role as far-left spokesperson to Bernie. She got a start on her career by claiming Indian heritage. She tried to put that behind her last Summer with a DNA test that not only flopped but angered real Indians. She's struggled to get traction with her proposals. He proposal for college loan forgiveness was criticized on all sided. Her live-cast last Winter seemed forced and unnatural. Her campaign manager resigned over her decision to reject corporate donations. She's also struggled to break into the double digits in polls. With neither cash nor wide-spread appeal and no message that resonates her candidacy is doomed.
Beto O'Rourke: His main attraction is that he attracted a record amount of out-of-state donations in a losing Senate race. For a while the press loved him but they've moved on to Pete Buttigieg.
Pete Buttigieg: The mayor of South Bend Indiana came from nowhere to vault into the top contenders. He's already secured a spot in the debated based on the number of people who have contributed to his campaign. Buttigieg's main claim to fame is that he's openly gay. He uses this as a bludgeon, constantly insisting the Vice President Pence hates him for being gay. This is an outright lie. Pence has always been cordial to him. I suspect that Buttigeig's current popularity comes mainly from gays and he's already reached his ceiling. His record as mayor of a mid-sized city isn't that strong and doesn't really qualify him to be President.