Technically all you need to join the Horde is not to be hostile and swear oaths of loyalty. And I suppose proving you wouldn't be a burden.
For the Alliance you need to convince all races you should have a place with them. Something that requires shared values, a lack of bad blood, and either holding the same faith, or an appearance that isn't too unappealing. The Alliance is the harder faction to get into, for sure. Many races would be disqualified. Let's say the Gnolls got a new warlord with the capacity to unite the tribes again, and he wanted to throw his lot in with a side. The Horde would judge their merit, and ask for oaths of loyalty. I don't expect the Alliance to even consider it, unless this leader saved an Alliance race first to get them to plead on their behalf, somehow. Beyond that it's an uphill battle. Lack of civilization and intelligence would make it pretty impossible to join the Alliance as well.
But yeah, as others have said, there are costs to joining a faction. You will get dragged into the conflicts of your allies. It's why you probably won't see many races join whose interests are limited to small areas of the world. Joining a faction is generally something you do because you are already in a situation where your people could really use someone strong on their side.