I once worked at a utility studying this very issue.

The typical worries about solar are:

+Power intermittency (rapid, uncontrolled changes in power can push the voltage and frequency outside acceptable levels. This reduces reliability and can destroy electronics.)

+Voltage regulation (also, on a local level, PV can mess with voltage compensators, pushing voltage outside of acceptable levels at the end of voltage regulation zones)

+Morning power surplus (lots of solar power will drop the baseline demand from ~4AM to noon, reducing the amount of cheap baseline power you can run)

+Harmonics (crummy inverters can inject unwanted sine wave harmonics into the grid, again bad for electronics)

+Fault current danger (if a tree takes down a power line, both sides can be remain live if power is being pushed onto the grid by PV on both sides)

+Inverter tripping (the main way to fix the fault current problem is to mandate that inverters passively follow the grid's voltage/frequency and then disconnect if they detect problems. However, with high PV penetration this has its own problem: a transient fluctuation can cause a domino chain reaction where all the inverters turn off, taking the grid down with them.)

+Power factor/vars (one important function for the grid is to keep current and voltage in phase, but today's PV inverters are dumb and blindly produce power with a power factor of 1, which can mess with automatic capacitor banks meant to help counteract the inductive load of the grid)

Now, some of these problems are more real than others, and many don't become an issue until PV penetration reaches like 30%.

The good news is that these are all solvable problems. Energy storage or additional natural gas peakers can fix the intermittency problem, and for the other problems the grid can be redesigned to allow better two-way power flow and inverters can be designed to produce higher quality power.

However, the bad news is that all of these fixes are expensive and all of them take years. Furthermore, many utilities do not have the expertise or confidence to make big changes to their grid and still guarantee reliable high quality power. Utilities are known for being slow and risk-averse, but they are risk-averse for a reason - blackouts are very costly to society.

Another issue for the utility is how to charge for power. Historically, utilities have tried to keep things simple for residential customers by charging only for power consumed. However, if residential PV becomes popular, utilities will have no choice but to change their pricing model so that they can be compensated for the non-kWh value they provide (e.g., reliability, stability, option value, power quality regulation).

The bottom line is that the problems from high PV penetration are real. Utilities are cautious, not evil, and they have good reasons to be. (Also, I think it's ridiculous when people claim that PV will kill utilities - barring a radical shift in technology/economics, wires and pooled power regulation and economies of scale make a grid hugely valuable, even if power generation becomes more distributed.)

That said, these are all solvable problems and I am hopeful and optimistic about the long-term future of solar power.