What Lugia said. If you follow the Rack forum, you'll see a lot of peoples' "first" system being just a collection of "that looks cool" modules without any thought to what they'll be able to produce with their collection. That's not bashing anyone's first attempt or amount of knowledge, just stating some of the common traps people fall into.
Attenuators, attenuverters, traditional ADSRs, even LFOs seem to be overlooked in a lot of first builds... they aren't cool or flashy... but they are fundamental elements in almost any kind of build.
Another factor overlooked would be ease-of-use. There are some really powerful modules out there... but the learning curve... remembering all of the secret double, long hold, double button presses to access features will really bum you out. Mutable Instruments makes some quality stuff. But unless you're devoted to committing each of them to your day-to-day thoughts, you'll always find yourself referencing the manual. The Expert Sleepers Disting is one of those modules that packs a lot of features but the interface is horrible to get around in. Again, I'm not bashing anyone's choices... but they may be a factor in what modules you'll want to select.
Ergonomics. 2HP makes some space-saving useful stuff. But dialing in settings is hard as hell with those little pots. A lot of modular is finding that 1mm of "sweet spot" of knob turning. Good luck on a 2HP module crowded next to another 2HP module. DEPTH can also be an issue with space-saving modules (or any other module) as some racks can be compromised on depth. Always know the depth(s) of your rack. You'll also want to consider ergonomics with things like VCOs. A great example is choosing the pitch. One solution is to have a course tune knob that runs the entire frequency range and a fine-tune knob. Another strategy is to select octaves (using layman terms here) and have a fine tune knob for the frequencies in between. Both have their advantages and disadvantages. Compare the Tip Top Z3000 vs the Intellijel Dixie II+ regarding the two methods.