Most all SLA researchers agree that the implicit linguistic system, the mass of the iceberg below water, is developed ONLY via input. A comprehension-based method builds the iceberg from the bottom-up. Traditional instruction, with its focus on grammar and output, focuses on the tip. Whatever tip that traditional instruction manages to construct is quick to melt away.

“Consciously learned grammar is only available as a Monitor or an editor, and the constraints on Monitor use are severe: 
1. The user has to know the rule, 
2. have time to apply the rule, 
3. and be thinking about correctness.” 
- Krashen

“Instruction does not appear to influence the order of development. No matter what order grammatical structures are presented and practiced in the classroom, learners will follow their own 'built-in' syllabus.” – Ellis

The Frequency Flower: The outer circle represents the language whole. The further from the center, the lower the frequency of that language structure.

Comprehension-based, communicative approach will grow out of the high frequency words. It starts with the innermost yellow disc and expands, each successive disc incorporating the previous disc, thus ensuring frequent review. Grammar is not targeted and vocabulary acquisition is horizontal (unrelated sets).

Traditional instruction covers* the petals one at a time, with its focus on tense-based units and vertical vocabulary lists (related sets, e.g. colors: red, blue, etc.) with varying frequency. Words taught in vertical lists are more prone to being confused with one another. Despite the possibility of covering more language, there is much less recycling of the language, which causes a review problem. Not all of the petals get covered, creating gaps in essential language. 
*covers does not mean acquire

50 words = 60% of speech 
1,000 words = 85% of speech
Verbs = 20% of the language

Examples of the highest frequency and lowest frequency thematic words traditionally taught:

Why do we care so much that someone can count to ten or higher?
(72)one (56)two (119)three (188)four (231)five (375)six (470)seven (492)eight (786)nine (346)ten

Only the numbers one and two are in the most-frequently used 100 words. There are more than 300 more frequent words than the numbers 6 through 10, and the numbers 13 through 19 are not in the most frequently used 1,000 Spanish words.
Source: A Frequency Dictionary of Spanish (2006) by Mark Davies