I agree that communication is key. People think that just because they can talk (or write), they can communicate. Not so. The bigger half of communication is learning to listen and not to just the words but to what is really going on in the other guy. And then having compassion for it.
Commitment is key, too. There has to be a willingness to commit and actually "partner" with the other guy. You're in life together. So you're there to help one another with the inevitable problems of life.
I've been in both open and monogamous relationships, each has their positives and negatives. Either way, establishing clear boundaries through open, non-judgemental communication, is essential.
Maturity is another factor. I think it is harder for younger guys (in general, obviously it depends on the guy) to commit and communicate effectively.
Finally, social pressures also play a big part. Lets face it, growing up in a htero-normative world, we don't see long-term, committed gay relationships around us in our communities or shown in movies and such. With the very rare exception, we don't grow up knowing gay couples and seeing how they are with one another or having them as role models. Moreover, we don't have them as common and continuous social support. (One of my partners and I were in a gay couple's support group, meeting every third week, for over seven years -- with 7 other couples!) Where straights are 'expected' to get married, raise families and 'fit in' with society, there is no such social expectation for gay men. We, as individuals, may be out more or less but gays are still not truly integrated into the over-reaching culture and society in which we live.
So, its complicated. Personally, I think the most important thing we can do is be supportive of our gay brothers who are trying to make a life together.