Time Warner is a media conglomerate. AT&T is one of the largest telecom companies in the US. The fact that AT&T wants Time Warner in their pocket almost certainly means that they plan to leverage that glut of media options, and some ways that they could do that may harm consumers. Normally, there would be no question of the FCC getting involved in such a transaction, but the way that Time Warner holds their licenses and the looming question of which ones, if any, AT&T will get has cast doubt on whether the FCC is able to get a hand on this deal. Despite all of that, there is every indication that the likelihood of the FCC stepping in to do a public review is extremely high.
The main body of Time Warner itself only holds one FCC license for a station in Atlanta. All of the sub-brands, however, like CW, CNN, and WB, hold tons of licenses that allow them to work with cable TV providers across the nation via satellite. These licenses would seriously spice up the deal, and without them, AT&T is essentially only buying the rights to use Time Warner's owned media, but can't control its distribution. Transferring these licenses is where things get messy; right now, the two companies are talking out how they will approach the issue and what licenses, if any, AT&T will get. While most small transfers go through without a review, a large transfer or large number of them would trigger a review, as would simply bringing over the licenses in the $85.4 Billion deal.
While an FCC review seems imminent, it can still technically be avoided, at the expense of stripping AT&T of the meat of their prize. If it does happen, however, it won't necessarily spell the end of the deal. While the potential for abuse is there, like zero-rating Time Warner media on their provided web to damage media competitors or refusing or price-hiking Time Warner content for paid TV, the deal could still benefit consumers in some ways. Consolidation of AT&T and Time Warner as a one-stop content producer to consumer shop could, for example, allow innovative new ways to deliver content; not many would likely argue against Game of Thrones in 360 degree VR, so long as the deal is otherwise harmless. If the FCC invokes a review, they won't be the only ones determining how harmful a deal could be; there will be a time frame for public comment, and the two companies wanting to merge must prove that their actions would benefit consumers. It will be AT&T and Time Warner's bigwigs and lobbyists' word against that of FCC staff who have seen this sort of thing go sour, and have stopped such deals in their tracks, many times before.