A Romney presidency almost certainly means a much better showing in the House and Senate in 2012, so he'll have a firm margin to work with. It's likely he gets 3 Supreme Court nominees (Scalia, Thomas and Kennedy probably all retire on his watch, so they can be replaced by a Republican), so he'll also have a very friendly Court, as swing-justice Kennedy is replaced by a more reliable conservative.
The House is so gerrymandered that I don't see the Democrats retaking it in 2014; it would take a ridiculous wave (something on the order of a 7-8% margin, IIRC) to elect a Democratic majority. Both 2012 and 2014 were fairly poor years for the Democratic Senate map, so a significant Republican gain in 2012 probably flips the Senate, and means it will be harder for the Democrats to retake it in 2014.
Foreign policy: the US probably intervenes in Syria and gets more active against ISIS from day one; that could be either a good thing or a bad thing, depending on how it goes, but I'd bet on "quagmire." No Iran deal, which means Iran continues towards nuclear weapons; if the US is actively fighting in Syria and Iraq, they probably don't also invade Iran, but who knows?
Domestic policy will be a Republican wet dream; they have unified control of the government. Most of Obama's OTL domestic policy from 2011 on was done by executive order and rule-making due to Republican obstruction; obviously none of it happens (so no Deferred Action, no overtime rules, etc.). Obamacare is gone, but who knows what replaces it, if anything. Tax cuts and spending cuts galore.
Clinton still runs in 2016, and probably has an easier time winning the nomination. The incumbent advantage is probably strong enough for Romney to win reelection, however.