How strong was the 2012 field behind the top 4? Strength of field is always a difficult one to judge at this stage.
For example, the 2011 Vuelta featured defending champion Vincenzo Nibali, an Igor Antón out to avenge having lost the Vuelta through crashing out the previous year, when many (myself included) believed him to be the favourite, a Joaquím Rodríguez who had only lost out the previous year due to a disastrous TT but was rested and ready and targeting the race, a Michele Scarponi who had podiumed the Giro (was not yet the winner officially), a Van den Broeck who'd narrowly missed the podium in the 2010 Tour, salvaging his season after crashing out of the 2011 one, a two time former winner of the race (at the time) in Denis Menchov, the 2008 Tour winner and somebody who'd podiumed the race multiple times in Carlos Sastre, a Janez Brajkovič that some people still thought would become a star... but all of them either underperformed or turned out to be false hopes.
The 2011 Tour, on the other hand, was lauded by many thanks to its epic finale that meant people forgot that the first two weeks were a pile of steaming manure - and the excitement generated by Schleck, Contador, Evans and Voeckler plus supporting case in the final few days meant people didn't miss the many who crashed out and DNFed so much - van den Broeck, Vinokourov, Wiggins, Brajkovič, Horner, Klöden, Intxausti, Gadret and Poels all involved (on the other hand, the loss of Amets Txurruka in the early part of the race was immediately felt and perhaps explains why week 2 of that race was among the worst raced of all time).
Just listing the number of riders on the startlist with big reputations only takes us so far. At the Vuelta, especially in years where the Worlds is hilly, we've seen plenty of people use 'Worlds warmup' as a ready 'out' for if their form isn't there - take for example Damiano Cunego in 2009. He won on the Alto de Aitana, and looked strong, was in the top 10 overall, but when he had a bad day in the queen stage, he simply let go, came in with the autobus, so he could get in the break a day later, won the stage from that then quit the race, having 'achieved all he needed'. Realistically, he had gone with the GC in mind, but once he realised he wouldn't be able to fight for that, it was better to abandon all GC hope, get another stage win and then prepare for Mendrisio than to fight on, tire himself out and finish 8th. Big guns being eliminated early is always something that happens at Grand Tours (take Dumoulin at the Giro and those 2011 Tour riders as a good example, but we've also seen Nibali DQed from the Vuelta for the team car trying to move him from group to group, Froome and Contador at the 2014 Tour, Valverde breaking his leg in 2017, Quintana losing red and dropping out the next day in 2014 and so on), and is more common at the Vuelta due to its late-season position.
And that 2011 Vuelta that seemed so weak at the time, its podium doesn't seem so weak now that Froome and Wiggins are both Tour winners and Froome has gone on to win 6 GTs (on the road - 7 once that Vuelta is incorporated).