After reiability, which relates to "If you can't finish the race you can't win the race", handling is vitally important. And keeping the tires on the pavement is an important part of handling, a MOST important part. So now areodynamics is one very good way to hold the car to the road, even moreso since the rules don't allow the cars to carry gravity intensifiers or inertia absorbing systems. Presently the cars are l9imited to holding onto the track by means of down force, which is provided by aerodynamics.
The result is that with the limitations on horsepower, which is limited by engine rules, all of the cars are underpowered, at least by the explanation that I got from one driver years ago, " If you can make it all the way around the track without having to back off, you don't have enough power". I have observed that to be totally correct, and I have used that relationship to win some races. But my point is that with all of the cars being a bit underpowered, the only areas left are handling and reliability. And reliability is not exciting to watch.
As a racing enthusiast, the state of Indycar racing saddens me greatly. The Indianapolis 500 began as a race that encouraged innovation with a minmal set of rules [like a max engine size of 600 CID] and has evolved in the opposite direction. Many of the rule changes are for safety - like smaller engines. The most disappointing thing today is that as the rules continue to narrow, they support the status quo with whatever teams can squeeze out of it. The most blatant reaction was in 1967 when Parnelli Jones looked like he was going to win with 10 miles left and then had a bearing failure [oops - reliability problem]. Jones had qualified his turbine powered car in the top 10 and had shown that he could run ahead of everyone - the reaction of the rules committe could have been to 'tweak the rules' to equalize the turbine, but their reaction was to reduce the turbine intake area by 33% - effectively saying 'we do not want them turbines running here!'. They have been gradually moving to an "everyone races the same car" model ever since. They even want to restrict firmware updates.
OK, got that off of my chest.
With all of the restrictions, teams still have to tune the overall system. Although the cars 'all look alike' just like a casual observer might comment about the compact cars on a rental lot, there is more than one distinct body shape in the Indy photos [at least for this year]. Outside of the aero and mechanical reliability, it looks like evrything else is electronic - lots of opportunity for a tiny edge on the competition.
One thing that I envy about the guys that support these cars - REQUIREMENTS - the marketeers and chrome hangers aren't coming in every week asking for a new feature!!