1) His biggest success was keeping the country stable in very trying times, namely a time when Serbia was still a pariah state because Mladić and Karadžić were still at large. He also was far more intransingent when it came to ceding Serbian power and interests in Kosovo.

His biggest failure was a poor response to the 2008 economic crisis. It is understandable that small coalition partners needed to be happy in order for the government to survive, but completely folding to the Pensioner's Party's blackmail and raising pensions during a burgeoning economic crisis was a complete disaster economically.

Corruption is something every Serb complains about when their favorite party is not in power, so I wouldn't put too much weight in it, but of course anyone who thinks there wasn't plenty of corruption is deluding themselves.

2) The Radicals, i.e. SRS, always had a large support and repeatedly threatened to usurp power in the 00s being very close to a majority. This constantly forsed the former Democratic Opposition of Serbia parties who ousted Milošević into strange and unprincipled coalitions which soured the mood in their electorate very much. After a decade of stagnation, of enduring the growing pains of baby capitalism and of no increase in living standards, coupled with the incessant pressure and blackmail over the Hague and Kosovo from the west, many had simply enough.

The SNS branch of SRS realized the approach of the Radicals wasn't working, so they went from a radical to a moderate party, significantly changing their rhetoric and legitimately promising cooperation with the west. However, it still had it's 'nationalistic credo' intact, so it seemed at the time like the best-of-both-worlds party that very much appealed to centrist voters. Once the far-left abandoned Tadić because of his reluctance to bow to western powers over Kosovo, Tadić's fate was sealed.

3) Serbs always do this schtick. The current ruler is always 'the worst ever' and every single previous ruler gets reanalyzed in a positive way, even frikkin' Milošević who was an unmitigated disaster. When Koštunica was in power, he was the worst ever, then when Tadić regained power suddenly Koštunica's tenure was completely reappraised. Now it's the same thing.

My honest opinion is that there isn't that much difference between Tadić's rule and Vučić's. Both of them had/have a political program which appealed to the centrist electorate. Serbia is not as polarized as in the west where the lines between liberals and conservatives are sharply drawn. In Serbia, the electorate has very much felt burned by both the extreme right due to the wars of the 90s and the extreme left whose political program is openly directed by the west. Any politician who wants power in Serbia is going to have to appeal to the center.