As you've mentioned, there are a lot of ways to get gold. Generally focusing on one particular method will probably get boring quite quickly. Personally my preferred methods of gaining gold consisted of a mixture of the following;
Playing the market
This can be achieved by buying low and selling high, you're going to probably want addons like auctioneer to do this, so you can track market prices of everything on the auction house using the market scan functionality. Another deviation of this is in conjunction with tradeskills, for example buying items required as materials for alchemy, which when combined result in an item which sells for more than the sum of its parts.
This is the most consistent method of making gold easily is to play the market, and some of my favourite methods of achieving this utilise secondary tradeskills like cooking - buying materials to make food, where the food sells for a profit in comparison to the sum of the materials required to cook it, for example. Of course, all of this depends on the server you're on, many servers already have a guild or two who control the auction house, which will limit the amount of gold you can make out of playing the market like this.
Alchemy, jewelcrafting, tailoring, inscription... Many high end players will have access to these already, but for everybody else it's perfectly possible to make a decent profit by crafting items and selling them on the auction house. Generally the peak time to do this is at the start of each tier - I made a lot of money at the start of the expansion making Darkmoon Cards and selling them on the auction house, for example.
The profitability of utilising tradeskills after the initial few months of an expansion is going to depend heavily on whether you're on a high population realm or not, if the demand does not exist, and you're outside of the first couple of months of an expansion (or tier, in the instance of tailoring/leatherworking/etc) then this isn't going to make you an immense amount of gold.
Once you're outside of the first month or so of an expansion, it can frequently work out that the individual materials required for tradeskills (dusts, cloths, etc) might sell for more than the finished items, in this case - sell the raw materials and play the market instead.
One set of tradeskills that always work out as 'free gold' is the gathering subset of skills. Mining, herbalism and fishing materials are always needed, especially on a higher population realm, and while time consuming can lead to decent profits if you plan out what you're doing in advance. For example, I used to have a 'gathering character'. This character was a death knight - for the increased movement speed while mounted - but a paladin would suffice in this role. This character had mining and herbalism, and would fly around mining and herbing. I'd fill up my bags, and then mail the materials over to a character I had in a major city so that my gathering character never left the region that I would gather in (to save travel/setup time for the next time I wanted to gather).
You will make a decent amount of gold per day, but is time consuming and has a much lower gold per hour yield than just sitting in the auction house playing the market. Helps to break up the monotony of sitting around in a capital city though.
Gold from raid farming
Once you have your raids on farm and aren't wiping constantly, you can make a decent amount of gold from raiding the current tier. If you're able to solo older raid tiers, you can make several thousand gold quite quickly by looting everything you kill in say, Black Temple (as an example).
Reportedly, some people have made quite a lot of gold in a short period of time. There are even people who've made it to the gold cap on several characters. In many of these instances they're taking advantage of an opportunity - taking my earlier example of selling Darkmoon Cards in the first few weeks of Mists of Pandaria, this was a very profitable business for me and I did during these few weeks make many times the cost of the Grand Yak you've linked, but as you get further into an expansion the opportunity to sell Darkmoon Cards drops quite rapidly and the focus changes more to the new gear recipes that get released with each tier (these are normally raid drops).