Dynamic mics sound flat

Dynamic mics tend to be less sensitive... and don't typically capture all the subtle nuance that a condenser would.
Keep in mind that dynamic mics also vary
ie:  An SM57 doesn't capture much bottom or high-end... whereas a RE320 or SM7b captures more of both.
Listen to a voice recorded with a RE320.  Lot's of upper-mid articulation... that might make you think it was recorded with a condenser.
 
The dynamic range captured with a dynamic mic may not be as extreme as with a condenser.
With high-tranient material (drums, electric bass, etc), you my find that dynamic mics capture a more aggressive attack.
ie:  Record a bass cab (or large tom) with a LDC and a SM57.  The LDC will capture more bottom and top... but will probably have a softer attack.  The 57 will capture a nice focused/aggressive mid-range attack (but lacks bottom/top).  Mix the two mics together... and you've got a nice balance of all the above.
 
Use the best choices you have for each particular scenario.
Use each mic's strengths/weaknesses to your advantage.

Within some limits, the behavior of a mic is dependant on the mechanical design. With both a condenser and a dynamic, you have a pressure sensitive diaphragm that moves with changes in air pressure. The dynamic diaphragm is attached to a voice coil, basically a spool of wire, that moves inside a magnetic field, and the interaction between the wire coil and the magnetic field creates electrostatic pressure, or a voltage, which is the source of the signal you record or process.
 
In the condenser, there is a thin layer of conductive material attached to the diaphragm, ideally one atom thick, but thicker in actual practice. Phantom power charges the diaphragm conductor, and moving the charged diaphragm causes a similar charge on another coated diaphragm, and creates the signal voltage.
 
(As usual, I greatly simplify, and sometimes actually misstate the physics of these interactions, for the purpose of illustration.)
 
So you have the motor out of a Ferrari, in each mic. The diaphragm. Lightweight, built for speed. Now tie one of them to a piece of aluminum foil with fishing line, (the condenser mic) and tie the other Ferrari racing motor to an iron boat achor with a massive length of iron chain (the dynamic mic). It's going to take a lot longer to get that boat anchor moving, because it is heavy. Once its moving, it will take a good while to stop, too, and...its likely to overshoot some, compared to the lightweight aliuminum foil.
 
The aluminum foil will accelerate and decelerate, very quickly, but, it is a much more fragile system, a strong breeze can scramble it...lets just leave the details beyond that alone today, ok?
 
So...
 
GENERALLY...
 
You are going to get quicker and more accurate response out of the condenser, at the price of needing an external power supply, and in a much less durable mechanism.
 
The dynamic will be able to hammer nails into wood if you are short of tools in a pinch, and handle much higher sound pressure levels, at the price of giving away some of the high frequency response it doesn't have the quickness to handle, and possibly at the price of "smearing" some of the higher frequency content that it can handle.
 
Finally, remember that both of these designs have been around a LONG time, and during that time, new materials have been invented, computer simulation for design purposes has begun AND come of age, and designers have been working on ALL of these advantages and disadvantages in BOTH type mics, so the general rule doesn't apply to the degree it did in the early days, in fact, some designs now flip the generalities backwards, at the price of other, yet undiscussed, resulting compromises and tradeoffs.
 
Stick your dynamic in front of the guitarist's wall of Marshalls, turned up to 12, or a quarter inch off the kick drum head, or knock the corrosion off the battery terminals of your truck if it won't start after the gig when the parking lot is dead empty and Nowhereville has rolled up the sidewalks.
 
Take your condenser out of it's padded case in the smoke and dust free studio environment and shock mount it on an out of the freeway mic stand to record the airy harps of heaven along with your latest angel's pristine voice.
 
Once you have that down, flip em around (use a cheap condenser to begin with) and see what you can do with them. If you can get awesome vocals and harps out of a battery acid infested dynamic, you might just have a long string Grammys ahead of you.