One of the achievements for Skyrim is:
Complete 10 side quests
I have completed the game on two playthroughs now and I've not got this achievement. What quests qualify as "side quests", since they are apparently not misc. quests?
Take a look at this answer
A Side Quest is any quest which appears in your quest log with this artwork in it's header:
These are quests that belong to no faction, and are not part of the main quest line, but are more involved than a 'miscellaneous' or 'radiant' quest (go kill 10 bandits in a dungeon, or fetch me a book, etc.)
Sidequests, in this case, refer to quests that aren't part of the main story, and aren't radiant quests listed under the "Misc Objectives" page of your journal.
I believe the catch, in this case, is that it can't be for any of the major factions (like the Thieves Guild, Stormcloaks/Imperials, etc.) as they have their own quest-based achievements.
So what's left? A handful of mostly unrelated, non-random, non-radiant quests. Rather than attempt to list what quests count, I'm simply going to point you at the UESP, which you can look through on your own (not sure your stance on spoilers). To be specific, I'm talking about quests that fall into the "Other" category. (this is for Solitude)
Steam is telling me that 25% of Skyrim owners have achieved this, so I'm going to trust that the achievement itself is not bugged.
PS3Trophies (warning, spoilers) says:
What is a side-quest? Well, that’s a trickier question that it sounds… a side quest in Skyrim is a quest that is big enough that it’s not classed as a “Miscellaneous Objective,” a quest that isn’t a questline quest (i.e. main quest, guild quest and the civil war quests) or one of the many Daedric quests. It’s tougher than it sounds in all honesty and tricky to work out. Out of 86 quests (at last check), only 12 of those were side missions. As long as it doesn’t fall under the three points above though and is fairly substantial (they’re not short missions!), then it should register as a side-mission..
You will find a list of quests in the above-mentionned page.