1.  Seamless

Seamless rain gutters mean rain gutters without seams, splices or splits that keep it from being continuous.  I've seen allot of homes that have had some yahoo take a 50 to 70 ft stretch and cut it in half and put it end cap to end cap.

This is poor workmanship and I consider it as lack of knowledge and talent. We have hung gutters that have reached one continuous length of 97ft. 

2. Corners

Companies that buy corners also lack talent.  A box which is show in diagram 1a was outdated in the 70'^s. These are garbage and provide 3 - 4 different seams to leak. Strip mitres  Shown in diagram 1d is commonly used by 9 out 10 companies. These are also build for speed only, The installer uses a hacksaw to cut a 45^o cut on both ends of the receiving gutter and the strip mitre receives both ends and acts as a band aid for the corner. This is unsightly and causes more flex in the corner and has two seems that will likely pull apart in time.

Diagram 1b & 1c show what hand cut seam corners look like. One end is cut as a female end (smooth cut) and the male end has 1/2 to 1" tabs leaving only 1 seam to be sealed.  Good Companies know how to hand cut. This should be a must for your house.

3. Slope

Rain gutter should be sloped at a average of an 8th of a inch for every 10 ft. Gutters over the length of 35ft should have two downspouts.

4. Edge metal

The back of the rain gutter should be tucked behind the drip metal to insure a proper water tight install. Of course if the house has a problem with structure, such as improper fascia installation or a dipped roof line this will have a factor in the way the gutter looks at the reveal.  (see diagram 2a)

5. Covers

Check out Gutter Shell (Click here)

6. Heat Tape

Consult a electrician

7. Mounting Brackets

There are 2 systems of mounting that really work great.  First is the 7" nail & Ferrule which should installed 18 -22" apart.   (see diagram 2a)

Second system is the hidden hanger bracket they also should be installed 18-22" apart and are screwed in with at least a 2" wood fast water tight screw (see diagram 2b)

Metal Roofs require twice the amount of mounting brackets and usually need to be accompanied with sno-brakes mounted on the roof above the bearing wall.  (see diagram 2c)

8. Paint

If your house has gutters or your planning to install gutters make sure aerosol paints are not sprayed on any of the parts such as corners, down spouts, straps or scratches. As these will fade and the corners will have stripes. The screws should be pre-painted before being installed.