Ah, this is guaranteed to start a very lively debate.  But before my likes and dislikes, let me say that the AR-type rifles are probably the most copied and modified self-loader on the planet at this time.  Customization of AR-type rifles has become a cottage industry of late: (1) flattop receivers and gas blocks (for detachable sights); (2) side or telescopting butt stocks; (3) different calibers -- .308 Winchester, 6.8 SPC (Remington), and .223 Remington, .243 Winchester; (4) different length and weight and contour barrels -- short to long, light to heavy, smooth or fluted; (5) different kinds of handguards -- short or long, free-floating sleeve, with or without rails for accessories; (6) different kinds of sights -- scopes, red dot or halographic, laser designator, night vision, fold-down front and rear sights (for flattops and modified gas blocks); (7) different kinds of accessories -- bipods, front pistol grips (CTC makes one with a laser designator), flashlight mounts, laser sights, etc.; (8) different kinds of muzzle devices -- none, just a crowned and recessed muzzle for precision rifles, flash suppressor of many kinds and types, compensators, sound suppressors.  If you can think of it, you can build it or customize it.

 

Dislikes:

1.  Muzzle light in the AR-15/M16A1 rifles -- no natural point of aim.  The AR-15A2 and M16A2 cured this problem by the heavier barrels and symetrical round handguards.

2.  Plastic furniture -- the triangular (RH and LH) handuards in the AR-15/M16A1 rifles did not fit the hand and were a spare part problem.  The original AR-15/M16 butt stock was too short and lacked a butt trap for stowage -- this was cured in the AR-15/M16A2 by lengthening it 5/8-inch and adding a butt trap for stowing a cleaning kit.  A skinny pistol grip on all original equipment, all models.  I replaced those on all my rifles with a with thicker, contoured grip.  All of mine now have a trap in the bottom of the grip for stowage of small items like spare parts.

3.  Lousy windage adjustment for AR-15/M16A1 (using the point of a bullet).  This was cured by an aftermarket add-on for the civilian guns and for all by the adoption of the AR-15A2/M16A2 type rear sight.

4.  5.56 NATO (.223 Remington) round.  Suitable for small game, but does not have either knock down punch or range.  Accuracy is outstanding, but bullet does not "buck" crosswinds very well.  Heavier bullets and faster twists have helped in recent years, but the range and punch have remained a huge problem -- especially with the trend to go to shorter barrels in the M4-type carbines.  The solution here is the 6.8x43 SPC or 6.8 Remington.  There is also a 6.5 Grendel, but the 6.8 Remington has been introduced as a production cartridge, whereas the 6.5 Grendel remains a wildcat.  The U.S. military (Special Operations Command and Remington) collaborated on the 6.8x43 SPC with an eye to upgrading current 5.56 NATO (.223 Remington) legacy platforms to a newer and better cartridge. 

5.  Gas system sucks.  The legacy gas tube, bolt carrier key, and bolt with gas rings was a probem in the original design and remains a problem for all AR/M16/M4-types with this system (including clones).  Carbon fouling can be reduced by using DuPont IMR powders (they burn hotter and this was the kind of powder Eugene Stoner used in this basic AR-15 design), but these parts still get fouled.  There are several competing gas piston systems on the market, one of which can replace the standard system with a minimum of part replacement, the others require replacement of the gas block and bolt carrier, plus the addition of the new parts.  Some of the new gas piston systems are adjustable (a very nice feature) and some are not.  All the gas piston systems are superior to the gas tube-bolt carrier key system of the legacy AR-15/M16/M4.

 

Colt Specific Dislikes:

1.  The double headed screw that secures the upper and lower receivers.  Originally, this was designed to prevent the installation of an M16 upper receiver assembly on a semi-auto only AR-15 Sporter.  (That lead to a cottage industry solution of an adapter screw.)  While AR-15/M16/M4 clones have proliferated by a multitude of manufacturers since the first Colt Sporters were offered for sale in 1964, ALL of the clones have used the military captured pin design of the M16, with the exception of the Colt AR-15.  Colt uses this stupid arrangement to this very day.  Why?

2.  Redesign of the tigger and hammer pins.  For some silly reason known only to Colt's, they redesigned the trigger and hammer pins by increasing them from 0.125 inches to 0.156 inches in diameter.  This means that the lower receiver holes are larger and the hammer, trigger, and disconnector are redesigned to be Colt specific.  Everyone else making clones of the AR/M16/M4-series uses the smaller pins.  Why? 

3.  Redesign of the bolt carrier.  For some reason, Colt has had a fixation with the bolt carrier.  The first AR-15 bolt carriers and M16 bolt carriers were almost identical, except for a longer lower profile cut and recontour of the bottom by the firing pin to prevent the use of select fire M16 parts.  Both were chrome plated.  This was dropped after the military deleted the chrome plating requirement during early military production.  Some time during AR-15A2 production (when the larger trigger and hammer pins were introduced), Colt decided to extend the bottom cut on the bolt carrier all the way to the end.  The revised bolt carrier looks like a tuning fork.  The elimination of the bottom half of the bolt carrier where it butts against the butter tube, weakens the bolt carrier (my estimation) and looks like hell.  Colt probably did it to save money.  This is a rather moot point since Colt decided to only offer the AR-15 in rifle or carbine configuration through their Custom Shop!  What the hell???

 

"Ultimate Rifle":

The optimal AR-15/M16/M4 design would be one in 6.8 Remington (6.8x43mm SPC) and with a gas piston system (with or without gas regulator).  All of the features and add-ons of the original are retained while increasing reliability and lethality.  This is a win-win for everyone -- the "mouse gun" that roared!